The current study examined whether bystander behaviours in class were associated with being perpetrators of ethnic victimization and whether they moderated the association between disengagement from morality and perpetrating ethnic-based victimization. The sample included 1065 adolescents residing in Sweden (Mage = 13.12, SD=.42; 55% males) from the first wave of a three-year longitudinal study. Students completed self-report measures. The results showed that adolescents with high levels of moral disengagement had greater likelihood of engagement in ethnic victimization. At the classroom level, lower levels of defending intentions and higher levels of reinforcing behaviours were related to higher likelihood of engagement in ethnic victimization. None of the cross-level interactions between moral disengagement and classroom-level bystander behaviours were statistically significant. Together, these findings suggest that intervention programmes designed to reduce bias-based hostile behaviours in schools may focus on promoting defending behaviours in class. However, the findings also highlight that targeting social context (or at least bystanders in class) might not be sufficient by itself to intervene with morally disengaged adolescents' involvement in ethnic victimization. Intervention efforts may also benefit from including specific components targeting moral disengagement mechanisms.