Outcomes and follow-up: The primary outcome is the subjects' body mass index (BMI). Prespecified secondary end points include body composition, physical activity and sedentary behaviors, attitudes towards physical activity, and various cardiovascular risk factors. Surveys take place at baseline and annually at the end of each school year. Fasting blood is sampled at baseline and every two years. Physical examinations are performed by qualified professionals and questionnaires are administered to small groups of 3 to 6 students. All the procedures are standardized among schools and the interviewers are be regularly trained to assure the reliability of the measurements.