Health promotion programs benefit from an accompanying process evaluation since it can provide more insight in the strengths and weaknesses of a program. A process evaluation was conducted to assess the reach, implementation, satisfaction and maintenance of a school-based program aimed at the prevention of excessive weight gain among Dutch adolescents [Dutch Obesity Intervention in Teenagers (DOiT)]. Our process evaluation involved data collections by means of questionnaires among students, teachers, school board and site staff. The results indicated immense difficulties in the recruitment phase and therefore a low reach at school level. However, among adolescents of the schools that participated, the reach was high (84%). Furthermore, the classroom intervention was implemented successfully based on the number of lessons taught. Most teachers rated the DOiT-intervention positively; students rated the intervention 6.6 on a scale of 1–10. The majority of the teachers planned to implement the DOiT-intervention program in the future, as they perceived DOiT feasible for pre-vocational education students.