Prevention in School (PS) is a comprehensive program which aims to improve the learning climate and reduce problem behavior in elementary schools. Core components are teaching of school rules, praise and rewards to support prosocial behavior, and a forum involving parents. This trial investigated the effects of PS on its intended outcomes, and includes 23 schools, 3,207 students, and 188 teachers. Multilevel regression analyses indicated no program effects two years after the start of the program. Problems with program components, implementation and study limitations may have contributed to the null findings. The lessons learned may be used to guide program revisions, and are important to consider in future attempts to implement and evaluate similar school interventions in a Swedish context.