A Health Promoting School Intervention in 10th Grade ("My Life - I Decide"): Protocol for a Pragmatic Controlled Trial.

Background: Health promoting school (HPS) interventions have the potential to improve adolescent health and well-being, but evidence regarding implementation and system-level impact in real-world school settings remains limited. "My Life - I Decide" (My Life) is a systems-oriented HPS intervention developed to strengthen positive mental and physical health, school well-being, and health-promoting school practices among 10th-grade students in Denmark. Objective: This study aims to describe the intervention, study design, and evaluation framework of the "My Life" intervention and its pragmatic controlled trial, including effectiveness, process, and system-level evaluations. Methods: The intervention is informed by the World Health Organization HPS framework and combines curriculum-based health education, action-oriented teaching, life-psychological approaches, and education outside the classroom. The intervention includes four phases: (1) preparation through cross-sectoral collaboration between health and education sectors; (2) planning and local adaptation; (3) delivery of a health education program; and (4) anchoring of health-promoting practices at the school level. Effectiveness is evaluated using a pragmatic controlled waiting-list trial design. Four intervention schools (11 classes) were matched with 4 control schools (15 classes), including approximately 416 students aged 15-17 years. Primary outcomes include social and emotional competences, self-efficacy, mental well-being, health literacy, school connectedness, and student interpersonal relations. Student survey data are collected at baseline, postintervention, and follow-up, and effectiveness will be analyzed using multilevel mixed models. System-level impacts are assessed using a mixed-methods design, including school staff surveys, interviews with school and municipal stakeholders, and student focus groups. A realist-informed multimethod process evaluation examines implementation fidelity, acceptability, contextual factors, and mechanisms of impact across intervention schools. Data sources include observations, interviews, student registration, and postsession surveys completed by health consultants after each teaching session. Results: The study will generate quantitative and qualitative data on student outcomes, implementation processes, intersectoral collaboration, and development of health-promoting practices within schools. Findings from the effectiveness, process, and system-level evaluations will be triangulated to test and refine the initial program theory of the intervention. Recruitment of schools and student enrollment have been completed, and baseline data collection commenced in September 2025. Follow-up assessments are being conducted according to the study timeline. Qualitative data collection for the process and system-level evaluations were completed in June 2026. Conclusions: The "My Life" study will contribute knowledge on the implementation and evaluation of complex HPS interventions in real-world educational settings. The findings may inform future HPS initiatives and provide methodological insights into combining effectiveness, process, and system-level evaluations in adolescent health promotion research.