The effects of health-promotion lessons carried out in two senior high schools in the canton of Zurich were investigated in a controlled intervention study. The lessons were developed by teachers and students and integrated into regular school lessons. Of particular interest were effects on school climate, school stress experienced by students and their physical and psychological well-being. Students in the intervention and control schools were assessed with a comprehensive questionnaire three times over a period of 18 months. Health-promotion lessons took place in the intervention school between t1 and t2 (12 month interval). Results showed that participation in a structured research project with repeated assessments led to a critical evaluation of school climate and school stress. At the same time that students were more critically judging the school environment, their physical and psychological problems lessened. Results were the same in both schools, that is, there was no specific intervention effect. Presumably, participation in a prospective study with repeated assessments acted as a sensitisation and mobilisation of resources in adolescents. The study showed the possibilities for health promotion, however it also indicated conflicts between the duty of teaching and psychosomatic prevention strategies.