--- - |2+ There is very little research on the dynamics of peer groups in schools and aggressive behavior among Chinese youth. In the present study, we investigated the link between pure indegree centrality and aggression (i.e., overt and relational aggression), and examined moderating effects of moral disengagement on this association, among Chinese adolescents in classes with different percentage of aggressors. Participants were 589 sophomore students (446 boys, 75.72% of sample; Mage = 15.98, SD = 0.64) from 16 classes in one vocational secondary school. The results showed that the effects for overt and relational aggression were moderated by the proportion of aggressors within classes and moral disengagement. For students in classes with a higher density of aggressors, there was a strong positive association between pure indegree centrality and overt aggression (but not relational aggression) for those students with higher moral disengagement. Conversely, in classes with fewer aggressive students, the covariation between centrality and both overt and relational aggression was negative for students with higher moral disengagement. These findings are consistent with an ecological perspective on the development of aggression, which shows that aggregating aggressive students in school classes may strengthen the associations between peer dynamics, moral disengagement, and aggressive behavior. - Aggressive Behavior, Volume 44, Issue 6, Page 571-580, November/December 2018.