This article presents the process of constructing an effective self-assessment tool for monitoring health promotion in secondary schools (SS). The ultimate aim is to improve adolescents’ well-being and quality of life in their reference environment. The design and validation of the instrument were based on participatory action research (involving students, professors, parents, health professionals, youth workers, youth directors, youth managers). A mixed quantitative and qualitative approach was adopted. The construction process included the following five consecutive stages: (i) steering-group creation and definition of objectives; (ii) diagnosing students’ health and well-being in SS (case study); (iii) initial design and construction of health promotion indicators; (iv) validation based on expert judgement and (v) validation by means of a pilot test. The final construction of the self-assessment tool included 5 areas (healthy habits, affectivity and socialization, emotional well-being, safety and risks, and specific health situations), 9 objectives and 18 indicators deployed through a rubric. Each indicator is discussed in depth, specifying the most appropriate resources and health promotion activities for its application. The final self-assessment tool is designed to be sensitive to and effective for self-assessment of health promotion in SS. It contributes to improving adolescent health on an individual basis and also has an impact on the school environment. Finally, it also promotes analysis of the health content on the curriculum and the teaching-learning method(s) employed at the school. The self-assessment tool has been published open access and its implementation will continue to increase health promotion in SS in Catalonia.