The aim of the current study was to examine whether individual and classroom collective social-cognitive processes were associated with bullying perpetration among schoolchildren. Furthermore, the aim was to examine whether changes in these processes from Time 1 at grade 4 to Time 2 at grade 5 were associated with bullying perpetration change. Self-reported survey data were collected from 1344 Swedish students from 108 classrooms. Multilevel analysis found that individual and classroom collective moral disengagement was positively associated with bullying, and defender self-efficacy was negatively associated with bullying. The effect of individual moral disengagement change on bullying change was positive, and the effects of defender self-efficacy change and classroom collective efficacy change on bullying change was negative.