School bullying is a complex social and relational phenomenon with severe consequences for those involved. Most children view bullying as wrong and recognize its harmful consequences; nevertheless, it continues to be a persistent problem within schools. Previous research has shown that children's engagement in bullying perpetration can be influenced by multiple factors (e.g., different forms of cognitive distortions) and at different ecological levels (e.g., child, peer-group, school, and society). However, the complexity of school bullying warrants further investigation of the interplay between factors, at different levels. Grounded in social cognitive theory, which focuses on both cognitive factors and social processes, this study examined whether children's bullying perpetration was associated with moral disengagement at the child level and with collective moral disengagement and prevalence of pro-bullying behavior at the classroom level. Cross-level interactions were also tested to examine the effects of classroom-level variables on the association between children's tendency to morally disengage and bullying perpetration. The study's analyses were based on cross-sectional self-report questionnaire data from 1,577 Swedish fifth-grade children from 105 classrooms (53.5% girls; M_age = 11.3, SD = 0.3). Multilevel modeling techniques were used to analyze the data. The results showed that bullying perpetration was positively associated with moral disengagement at the child level and with collective moral disengagement and pro-bullying behavior at the classroom level. Furthermore, the effect of individual moral disengagement on bullying was stronger for children in classrooms with higher levels of pro-bullying behaviors. These findings further support the argument that both moral processes and behaviors within classrooms, such as collective moral disengagement and pro-bullying behavior, need to be addressed in schools' preventive work against bullying.