 A cluster randomised trial implemented in 24 high and technical schools including 20 000 fifth-former aims, on a three-year period, at evaluating the effectiveness of three strategies, combined or not, according a 222 factorial plan, concerning the promotion of healthy dietary and physical activity behaviours: to develop individual skills by individual and collective education, to propose favourable environment, to screen and care for subjects at risk of weight excess. Main outcomes are improvement of knowledge, attitudes and behaviours, mean body mass index BMI change, proportion of obese children according to international standards after two years of follow-up, and finally psychological and sociological factors which are facilitators or barriers to such actions. The evidence of the effectiveness of one or the other of the strategies will help to define interventions framework for medical, educative and economic sector actors. 