Adolescence is a key period for shaping lifelong health behaviors and positive development [1,2]. However, this stage is often characterized by declining well-being [3,4] and increasing health risk behaviors [5,6]. These patterns are strongly socially patterned, as adolescents’ health and well-being are significantly influenced by their parents’ socioeconomic status (SES) (eg, income, education, occupation, and social position) [7,8]. Adolescents from families with low SES face a higher risk of mental health challenges [9,10], engage more often in health risk behaviors [7,11], and often have fewer health assets [12,13] than adolescents from families with high SES [14]. Such health-related disadvantages are especially problematic during the transition from compulsory schooling to higher education or the labor market, increasing vulnerability and the risk of school dropout [15,16].

In Denmark (DK), one example of adolescents in the transitional educational phase is students attending the optional 1-year municipal 10th grade. Students in municipal 10th grade are more likely to come from families with low SES compared with peers who transition directly to upper secondary education [14]. They also report lower well-being and more challenges related to educational engagement and readiness [17], thereby increasing the risk of school dropout [18]. While empirical evidence about this group’s health behaviors remains limited, broader social inequalities are expected to affect their health and well-being, underscoring the need for health promotion interventions in municipal 10th-grade school settings.

Schools engage all students regardless of SES or ethnicity, making them key settings for health promotion and for reducing health inequity [19-21]. Evidence suggests that school-based interventions addressing system-level changes are particularly important for supporting students from families with low SES [22]. The most prominent is the systems-oriented World Health Organization Health Promoting School (HPS) framework [23]. This extends beyond health education to include structural changes in schools’ physical and social environments and collaboration with parents and/or local communities. The HPS adopts a whole-school approach with three core components: (1) formal health education, (2) a supportive school ethos and environment, and (3) engagement with families and/or the wider community [24]. Studies show small but significant positive outcomes of HPS interventions, including modest improvements in physical activity, fruit and vegetable intake [25], mental well-being [26], social and emotional competences [27], and bullying [28]. Some studies also suggest that HPS approaches may be effective in promoting health literacy or health-related action competence [27]. Yet, evidence is limited on the effect of school-based health promotion approaches, such as the HPS, on adolescents at risk of poor mental health and health behavior due to socioeconomic factors [29].

In Europe, the HPS approach emphasizes student participation and action-oriented teaching as forms of “Democratic health education” highlighting that health education is most effective when teaching is experiential, action-oriented, based on a holistic health concept, and participatory [30]. Evidence shows that such approaches within HPS can promote social and emotional competences [31] and health-related action competence [32], while similar student-centered and active learning approaches are associated with improved mental health and well-being among adolescents in secondary education [33]. Several didactic methods build on these principles, including the life-psychological method and education outside the classroom (EOtC). The life-psychological method emphasizes participation and actions through hands-on exercises and reflections and has been identified as a potential HPS method to promote well-being with promising results among youth in educational settings [34,35]. EOtC relocates parts of curriculum teaching to settings outside the school—nature and culture environments and societal institutions—linking classroom content to real-world learning environments. EOtC conducted in natural environments is highlighted as particularly effective in supporting student health and well-being outcomes [36]. EOtC more generally has been found to support school motivation [37], physical activity [38], and school-well-being [39], and EOtC aligns with HPS by emphasizing local community involvement and participation [40]. While these methods show promise for promoting health for young schoolchildren, their effectiveness in secondary education—particularly among adolescents from lower SES families—remains scarce. Furthermore, little is known about mechanisms that influence the outcomes in these groups. Altogether, the lack of evidence limits our understanding of how HPS interventions can address social inequalities in adolescent health.

Despite its potential, the implementation of HPS approaches is often challenged by the complexity of school systems and constrained by implementation barriers such as competing academic demands, limited time and resources, and insufficient organizational support [41,42]. These challenges often lead to partial implementation of interventions [43], thereby limiting their effectiveness and sustainability [44,45]. This underscores the importance of supportive and context-sensitive implementation strategies in the field. Strengthening intersectoral collaboration between the health and education sectors at the municipal level may represent a relevant strategy to support the implementation of HPS interventions. In the Danish context, this is particularly relevant, as municipalities are legally responsible for prevention and health promotion, including school health promotion and education. However, health consultants are often disconnected from the school health education curriculum, and teachers frequently lack the necessary competence in this field [46]. A potential solution to secure the necessary human and financial resources is to strengthen the role of municipal health consultants in supporting implementation. Combined with “add-in” approaches that integrate health promotion into existing curricula and align with the core tasks of the school, such strategies may support more successful implementation of HPS interventions [46,47].

To promote health and well-being among Danish students in a transitional educational phase and overcome known implementation barriers, the HPS intervention “My Life – I Decide (My Life)” was developed. “My Life” is expected to promote positive mental and physical health and school well-being among 10th-grade students (aged 15‐17 y) and strengthen health promotion capacity in schools and intersectoral collaboration. The intervention seeks to achieve these aims through 4 intervention phases, operating at both the student and system levels. The aim of this study protocol is to describe the “My Life” intervention and the design of the pragmatic controlled trial evaluating its effectiveness, implementation processes, and system-level impacts in Danish 10th-grade schools. Specifically, the study evaluates: (study 1) effectiveness on student health and school well-being outcomes; (study 2) impacts on school health promotion capacity and intersectoral collaboration; and (study 3) how implementation mechanisms and contextual conditions influence outcomes across school settings.

The intervention is theoretically inspired by the HPS approach [24], action-oriented teaching [48], the life-psychological method [49], and EOtC [36]. Following common HPS definitions [25], “My Life” uses a whole school approach and operationalizes the 3 HPS components through 4 intervention phases. The phases are: (1) preparation: establishing collaboration between the school and municipal health department and building implementation capacity; (2) planning: adapting the program to schools; (3) health education program: classroom and community-based activities; and (4) anchoring: sustaining health-promoting work at the school. These phases are described in detail below and illustrated in Figure 1 (see also the Template for Intervention Description and Replication [TIDieR] checklist in Checklist 1). Municipal health consultants deliver the intervention in collaboration with school teachers and local actors, aligning with national priorities to strengthen collaboration between the health and educational sectors [46]. The intervention adapts to the school context with preparation and planning taking place before the summer break and school start, while the health education program and anchoring are implemented after the summer break through the full school year. The intervention serves as a framework for HPS practices that can be anchored at the school and further developed over time.

During the preparation phase, a formal collaboration agreement is established between the school and the municipal health department, outlining roles, responsibilities, political support, and resource commitments. To support implementation, a 2-day training course for health consultants is provided. This covers training in applied didactic methods and the application of the HPS framework. During the training, health consultants receive a comprehensive intervention manual developed to support program implementation. The manual is also provided to the schools for them to understand and be inspired by.

This phase comprises three core activities that focus on organizing and tailoring the program to the local school context.

The health education program is integrated into the 10th-grade curriculum as an “add-in” [47]. It comprises 15 sessions delivered at the class and community levels, over 11‐13 weeks early in the school year. The sessions cover 12 topics related to physical and positive mental health and school well-being (see Table 1). Most sessions occur in settings outside the school following the principle of EOtC. Health consultants facilitate the sessions, including supporting and motivating students and coordinating with community partners. Teachers participate and support students who need additional support. In the final session, health consultants collect students’ feedback from the program to be presented in the Anchoring phase. Teachers and health consultants are expected to continuously discuss how to integrate intervention methods into teaching practices to anchor the program within schools.

This phase comprises one main activity at school and community levels. An anchoring meeting is held at each school after the health education program. This is facilitated by health consultants who invite teachers, school and municipal management, relevant community actors, and students. Student feedback and experiences from the health education program are presented. This is expected to initiate a collaborative process to develop health-promoting actions for implementation. These actions are intended to build HPS capacity in schools. Actions may be student-driven, based on themes or needs identified by students, or system-driven, initiated by the school or municipality, such as integrating health topics across subjects or introducing structural initiatives.

The intervention is underpinned by a program theory developed through a practice-research partnership between Vallensbæk Municipality, Steno Diabetes Center Copenhagen (SDCC), and the Intersectoral Prevention Laboratory [50]. The program theory outlines the final synthesis of the 4 main intervention phases, expected mechanisms of change, contextual factors, and expected outcomes at both the student and system levels. It illustrates how the intervention phases are expected to promote students’ physical and positive mental health and school well-being, health promotion capacity, and collaboration between school, municipality, and local community (Figure 2).

The development of the program theory guided the identification and selection of outcomes. Outcome selection was informed by both theoretical and empirical relevance, as well as stakeholder input, following recent recommendations for evaluating complex interventions [51]. Several primary and secondary outcomes were identified during the development of the intervention through qualitative data from students, health consultants, and 10th-grade teachers from Vallensbæk municipality, which was synthesized with existing evidence. At the student level, outcomes were grouped into three domains: positive mental health, physical health, and school well-being. Positive mental health was defined broadly to reflect students’ overall subjective well-being and psychological functioning [52], while school well-being refers specifically to students’ social relations and sense of belonging in the school. Physical health was addressed through health literacy as the primary outcome and several secondary indicators (health behaviors). This reflects the intervention’s focus on students’ competence and motivation, with health literacy understood as a key precursor for health-related behavior, whereas positive mental health and school well-being encompassed multiple subdomains. At the system level, outcomes were grouped under HPS practice and capacity, relationships and collaboration between the school, municipality, and local community, and health-promoting actions.

The main settings are municipal 10th-grade schools and surrounding environments, including municipal health departments and various community organizations, such as fitness centers, scout groups, and town halls. Municipal 10th-grade students are the primary target group. In Denmark, compulsory schooling ends in the 9th grade. Students not yet prepared for upper secondary education have the option to enroll in 10th grade, a voluntary transitional year focused on academic and social growth with no admission requirements defining eligibility [1]. The 10th grade aims to boost students’ educational preparedness through academic enhancement, elective courses, and social and emotional development [2]. Hence, this context may suit the implementation of innovative health promotion interventions like “My Life.” Students attend daily from home without additional costs, unlike 10th-grade students at residents’ schools. The secondary target groups are school and municipal systems, where the intervention aims to strengthen health-promoting school practices and encourage collaboration.

Evaluating system-oriented HPS interventions such as “My Life” is challenging due to their multiple target groups, settings, and phases implemented at different organizational levels [51]. Consequently, the evaluation requires multiple context-adaptable designs and methods to address the 3 research objectives [51] (Figure 3). The different designs address the distinct research questions and are analytically complementary, capturing effectiveness, system-level impact, and implementation processes [53]. The evaluation encompassing all 3 research objectives will take place from January 2025 to June 2026. The effectiveness trial (study 1) follows a pragmatic controlled waiting list design with the participating schools as the assigned unit of allocation. Pragmatic trials are designed to determine intervention effects under real-world conditions rather than highly controlled efficacy trials [54]. The design and reporting of the trial are informed by the CONSORT (Consolidated Standards of Reporting Trials) extension for pragmatic trials. Schools are assigned to intervention either in 2025-2026 or 2026-2027. Schools scheduled to receive the intervention in 2026-2027 serve as controls in the trial. The system impact evaluation (study 2) and process evaluation (study 3) acknowledge that HPS interventions are context-bound and are approached through system thinking and realist evaluation. These approaches provide nuanced insights relevant for discussing potential scalability and for understanding outcomes and effects beyond the individual level (Bonell et al [55]: will it work here and for whom?). System-level impact (study 2) is evaluated through a mixed-method design inspired by Pinzon et al [56], while implementation processes at intervention schools (study 3) are examined using an adaptive multimethods design informed by realist evaluation [57], systems thinking [58], and guidance for developing and evaluating complex interventions [51]. The specific methods, data collection, and data analysis used to answer the 3 objectives are explained in detail below and illustrated in Table 2.

Before initiation of the “My Life” partnership, Vallensbæk municipality collaborated through the regional partnership “Cope with Life – Without Smoke and Drugs” [64] focused on adolescent health promotion. This partnership provided a practical framework for recruitment. Schools were recruited using convenience sampling, as participation was primarily limited to municipalities and 10th-grade schools already in the partnership. Through this partnership, 9 municipal 10th-grade schools were approached. Vallensbæk Municipality, which had implemented an earlier version of “My Life” for 5 years, served as the pilot school of the fully developed intervention. Of the remaining 8 schools, 7 agreed to participate following introductory meetings, while one declined due to a lack of interest. To balance intervention and control schools and account for anticipated student dropout, an additional control school from the region was recruited. During intervention development, randomization of schools was deemed unfeasible due to practical and organizational constraints, including partnership-based implementation context and differing readiness among schools to implement the intervention during 2025-26 or 2026-27. This is consistent with recommendations supporting nonrandomized designs when highly controlled conditions are not feasible [51]. Instead, schools were matched based on (1) their number of students and classes; (2) contextual and sociodemographic factors (including percentage of students with Danish ethnic backgrounds and average grades in mandatory exams); and (3) readiness to implement the intervention during the school years 2025-26 and 2026-27. Because schools were not randomized, baseline differences between intervention and control schools are expected. To assess potential selection bias, baseline student characteristics and school-level contextual characteristics will be examined descriptively across groups. Relevant baseline covariates, including gender, socioeconomic indicators, migration status, and school-level contextual factors, will be included in the mixed-effects analyses where appropriate. Four intervention schools with 11 classes were matched with 4 control schools with 15 classes. All enrolled 10th-grade students in participating classes were eligible to participate in the effectiveness trial, except students who could not read and understand Danish. All students were eligible to receive the intervention.

Because the study was implemented within an existing municipal partnership structure, the sample size was determined pragmatically by the number of participating schools eligible for implementation during the study period. Based on expected enrollment across the 8 schools, approximately 416 students are expected to participate. Although no formal a priori power calculation was performed during intervention development, sample size considerations were informed by comparable school-based quasi-experimental studies targeting adolescent health literacy and school well-being outcomes, which have demonstrated statistically significant intervention effects with similar sample sizes [65]. The study should therefore be considered adequately powered to detect small-to-moderate intervention effects at the student level, while acknowledging that statistical power for school-level effects may be limited due to the relatively small number of schools.

The “My Life” effectiveness trial was registered in ClinicalTrials.gov NCT07411677. The study was reviewed by the Research Ethics Committee for Science and Health in the Capital Region of Denmark, which determined that formal ethical approval was not required under Danish legislation regulating biomedical research (reference number F-25020538). This decision was based on the study involving questionnaire surveys, interviews, focus groups, and observations without collection of biological material or biomedical interventions. Written informed consent will be obtained from all participants prior to data collection, including students in surveys and FGI, and school and municipal staff in interviews and surveys. Under Danish regulations for nonbiomedical research, adolescents aged 15 years and older may independently provide consent for participation in questionnaire- and interview-based research [79]. Parents and guardians were informed about the study through school communication systems before data collection and had the opportunity to contact the school or research team with questions or concerns. Parents or guardians were informed via school communication, and parental consent was not required under national regulations for nonbiomedical research. Parents or guardians of participating students were informed about the study through school communication channels, and in accordance with national regulations for nonbiomedical research involving adolescents, parental consent was not required. Videos explaining “My Life” and the implications of giving consent were shown in classes before collecting student consent. Data will be collected, processed, and securely stored in accordance with rules and regulations of the Danish Data Protection Agency. The project follows the Danish Code of Conduct for Research Integrity and the Helsinki II Declaration principle.

The “My Life” study will provide new knowledge on the implementation and evaluation of a systems-oriented HPS intervention targeting older adolescents in Danish 10th-grade schools, including an updated program theory revealing key mechanisms of change. Adolescent mental health and school well-being are shaped by complex interactions between individual, social, educational, and organizational factors, and HPS approaches have been suggested as relevant strategies for addressing such complexity [25]. The intervention will be implemented in real-world school settings characterized by diverse student needs and varying organizational capacities and will provide critical insights relevant to similar educational settings.

The “My Life” intervention combines curriculum-based health education with broader HPS principles, including intersectoral collaboration and local anchoring processes. A major strength of the study is the integration of effectiveness, process, and system-level evaluations within a single evaluation framework. This enables examination of both student-level outcomes and the contextual and organizational processes influencing implementation.

Another strength is the pragmatic design, allowing the intervention to be implemented under real-world conditions in collaboration with schools and municipalities. The close collaboration between researchers, municipalities, and schools during intervention development and implementation may strengthen contextual relevance and future scalability. Furthermore, the extensive use of qualitative and quantitative methods across multiple stakeholder groups, including students, teachers, school managers, and municipal health consultants, may provide a broader understanding of how HPS interventions are implemented and adapted within educational systems.

Potential challenges include variation in implementation across schools due to differences in organizational capacity, management support, competing educational priorities, and local adaptation of the intervention. While flexibility is consistent with HPS principles and pragmatic trial methodology, it may also influence implementation fidelity and comparability across schools.

In addition, sustaining health-promoting activities beyond the educational program itself may be challenging due to limited time and resources within schools and municipalities. Another challenge relates to the evaluation of systems-level change, as organizational and cultural changes within schools have been found to take more than one school year [28] and thus may extend beyond the study period. Finally, because the study uses a nonrandomized pragmatic design, there is a potential risk of selection bias compared with randomized controlled trials. However, the pragmatic approach was chosen to ensure feasibility and relevance within existing municipal and school structures.

In conclusion, the “My Life” study may contribute important knowledge about how systems-oriented HPS interventions can be implemented and evaluated in transitional educational settings. The study may inform future development of school-based health promotion initiatives targeting older adolescents and strengthen understanding of how contextual and organizational conditions influence implementation processes and school-level change. Furthermore, the study may contribute methodological insights relevant for future evaluations of complex public health interventions implemented within real-world school systems.