This study examined whether individual and collective moral disengagement (MD) in seventh grade were associated with bullying perpetration across seventh and eighth grade, and whether changes in individual and collective MD from seventh to eighth grade were associated with concomitant changes in individual-level bullying perpetration. In this short-term longitudinal study, 1232 students from 96 lower secondary classrooms answered a web-based questionnaire on tablets during school, once in seventh grade and once in eighth grade. According to the findings, and in line with the study's hypotheses, students who scored higher in individual MD in seventh grade and students who belonged to classrooms with higher levels of collective MD in seventh grade were more inclined to engage in bullying perpetration across seventh and eighth grade. In addition, students who increased in individual MD or belonged to classrooms that increased in collective MD from seventh to eighth grade reported increased levels of bullying perpetration. In contrast, students who decreased in individual MD and who belonged to classrooms that decreased in collective MD declined in their levels of bullying perpetration from seventh to eighth grade.