This follow-up study with 1,167 primarily White adolescents (aged 13.45 years at T1, 613 females) examined the impact of self-efficacy for aggression, moral disengagement, and collective efficacy beliefs on peer aggression in schools. Students completed questionnaire measures at the beginning and end of the school year (8 months apart). High aggression efficacy and moral disengagement scores predicted higher frequency of peer aggression over time. Low collective efficacy beliefs regarding the ability of students and teachers to collaboratively act to inhibit peer aggression were also associated with more frequent aggression, although this association was stronger at higher levels of moral disengagement. The findings of this study highlight the need to consider collective efficacy beliefs in conjunction with individual social cognitive processes when seeking to explain aggressive behavior.