As children reach adolescence, peer groups gain special power to influence their behavior, and young people tend to adapt their behavior to fit group norms. When students engage in bullying, alcohol and drug abuse, and other behavior, those in their peer group are more likely to engage in those behaviors as well. The authors sought to evaluate whether having students engage in peer learning through interdependent cooperative groups might disrupt this tendency. Middle school teachers were trained in using peer learning to teach their regular curriculum, and after one year, students showed more positive and less negative behavior than those in control schools that did not implement peer learning. The effect was also larger than has been reported for traditional prevention and social-emotional learning programs.