Background <p>Health is a&#xa0;central topic in contemporary physical education, with the promotion of sport-related health competence becoming increasingly important. The aim of this study was to evaluate the effects and processes of a&#xa0;participatory school intervention to promote competence among pupils.</p> Methods <p>In a&#xa0;quasi-experimental mixed-methods design over 18&#xa0;months, health-related teaching examples were developed at eight secondary schools (four intervention and four control schools) as part of a&#xa0;cooperative planning process (cocreation) involving physical education teachers, pupils, school administrators, and researchers. Effectiveness was measured using a&#xa0;standardized, validated test of sport-related health competence at two measurement timepoints, while the planning processes were evaluated qualitatively through guided interviews with physical education teachers.</p> Results <p>The results show a&#xa0;general improvement in sport-related health competence across all groups, with pupils from the intervention schools achieving significantly higher gains than those from the control schools (small effect). The cooperative planning processes were evaluated positively for the most part but proved sensitive to contextual factors such as support from the school management, the involvement of relevant stakeholders, and structural conditions.</p> Conclusion <p>Overall, the results highlight the potential of participatory approaches for the development of health-oriented and competence-based physical education but at the same time reveal challenges in implementation under different school conditions.</p>