The onset of puberty is considered a critical period for the development of overweight and obesity. For prevention purposes, we developed the school-based intervention program TOPP (Teenage Obesity Prevention Program), especially for boys. In order to test the effectiveness, we conducted a controlled study using a pre-post design. A total of 84schools in Thuringian, Germany, with 1,199boys participated in the study. Program effectiveness was analyzed with mostly standardized questionnaires referring to body-related self esteem, eating behavior, physical activity, teasing, and knowledge. The program was performed during the course of a school project within at least 3weeks or during the regular school lessons for more than 6weeks. After 9×90-minute, manual-based lessons, including interactive exercises and poster-based group discussions, significant improvement was only reached for nutritional knowledge. As a main outcome, it could be demonstrated how an area-wide prevention program with low costs could be successfully implemented. The school environment enables us to create a universal, socially equitable, and low-threshold access.