This article focuses on the link between social norms and behavioral development as presented in research on norms regarding bullying and aggression. The aim is to present a conceptual framework for how classroom norms may explain children's decisions to defend others or refrain from defending. Norms emerge from group consensus about what is appropriate in given social circumstances, and can also shape, constrain, and redirect behavior at the individual level. The study of norms has gained much attraction in peer relation research, and has turned attention to group-level processes, often defined at the classroom level, which create and sustain shared meanings that impact behavioral and social adjustment. Norm conformity, pluralistic ignorance, and power balance are presented as potential micro-level mechanisms for the link between classroom popularity (or rejection) norms and defending behavior. Directions for further research are discussed, including the need to assess and test the microfoundations directly, examine gender-specific versus common norms, focus on competing classroom norms, test developmental effects of norms, examine the impact of teachers on social norms, and pay attention to the influence of personal norms.