When school-based prevention programs are put into practice, evaluation studies commonly only consider one indicator of program implementation. The present study investigates how two different aspects of program implementation – fidelity and participant responsiveness – jointly influence proximal outcomes of the school-based violence prevention program ViSC. The program was implemented in 20 schools across Austria. Trainers conducted in-school teacher trainings with varying implementation fidelity to the program schedule. In a two-wave evaluation study, 370 teachers provided data about their participant responsiveness and the program’s proximal outcomes. The latter comprised their self-efficacy to stop violence among students and their behavior in bullying situations. Multilevel analyses showed that teachers’ self-efficacy was significantly more enhanced in schools where the ViSC program had been implemented with high fidelity. Furthermore, only teachers with high participant responsiveness significantly changed their behavior in bullying situations. Implications for program developers and suggestions for further research on implementation are discussed.