We conducted a three-arm cluster randomized controlled trial (n = 2491) to evaluate a teacher delivered intervention to reduce aggressive and antisocial behavior and increase prosocial behavior in the classroom. A second aim of this trial was to establish whether combining this intervention with an intervention for parents was better than the teacher-only intervention. This evaluation was conducted in a resource-poor setting (Pereira, Colombia). The findings show that both interventions prevented increases in aggressive and antisocial behavior. However, the teacher-only intervention also had a positive impact on increasing prosocial behavior. (Contains 3 figures and 3 tables.)