Population and design: The Intervention Centered on Adolescents' Physical activity and Sedentary behavior (ICAPS) is a 4-year randomized prevention study targeting 6th graders (e.g. 12 year old adolescents). The study design is based on a randomization at school level and follows the intention-to-treat principle. Eight schools (four intervention schools and four controls) are randomly selected out of the 77 public middle schools of the Department of Bas-Rhin (Eastern France). To balance potential confounding factors and to ensure a broad socioeconomic context, randomization is done after stratification on sociogeographical criteria. Intervention status of the schools is randomized in each pair of matched schools. All initially first-level students of the randomized schools are eligible to participate.