This study examined whether the effect of social group norms on 7- and 10-year-old children's aggression can be moderated or extinguished by contrary school norms. Children (n5384) participated in a simulation in which they were assigned membership in a social group for a drawing competition against an outgroup. Participants learnt that their group had a norm of inclusion, exclusion, or exclusion-plus-relational aggression, toward non-group members, and that the school either had a norm of inclusion, or no such norm. Findings indicated that group norms influenced the participants' direct and indirect aggressive intentions, but that the school norm moderated the group norm effect, with the school's norm effect tending to be greater for indirect vs. direct aggression, males vs. females, and younger vs. older participants. Discussion focused on how school norms can be developed, endorsed, and presented so that they have their most lasting effect on children. Aggr. Behav. 36:195-204, 2010.