This study investigates the effectiveness of the PRIMA antibullying program for elementary education using a cluster-randomized trial with two experimental conditions (with and without student lessons) and a control group. Students of 31 schools participated in the study (N = 3,135; Mage = 10 years). Multilevel regression analyses demonstrated positive effects of the program on peer-reported victimization and reinforcing behavior. Implementing multiple program components was related to stronger program effects. The results provide partial experimental evidence for the beneficial effects of combining student lessons and teacher training in antibullying programs. Future experimental research is needed to investigate other approaches that reduce not only peer-reported victimization, but also self-perceived bullying and victimization.