The purpose of this study was to evaluate the effects of a pilot school-based intervention called CEPIDEA, designed to promote prosocial behavior in early adolescence. The study took place in a middle school located in a small city near Rome. The intervention group included 151 students (52.3% males; M_age = 12.4), and the control group 173 students (50.3% females; M_age = 13.0). Both groups were assessed at three time points, each 6 months apart. A Latent Growth Curve analysis revealed that the intervention group, compared to the control group, showed an increase of helping behavior along with a decrease of physical and verbal aggression across time. Current results also showed that the increase of helping behavior mediated the decline of verbal aggression in adolescents who had attended the intervention. Participants of CEPIDEA also attained higher grades than the control group at the end of middle school. Overall, findings suggest that promoting prosocial behavior may serve to counteract aggressive conduct and enhance academic achievement during adolescence.