[en] Although the majority of students disapprove of aggressive behavior in school, only some of them intervene when witnessing aggression between their classmates. This study examines how bystanders might be activated to support the victims of aggression in school. The effectiveness of a Bystander Intervention Training for students, implemented via teachers, was evaluated. Data from 22 training and 26 control classes were collected at three occasions and analyzed using multivariate multilevel analyses. The primary evaluation criterion was the change in frequencies of aggression and active vs. passive bystander behavior. The training reduced passive bystander behavior and victimization over time. Training effects were stronger for classes with high implementation quality. Recommendations for the implementation of the training will be discussed.