<p>School bullying is a complex social phenomenon that is embedded in the classroom. At the class level, classmates and teachers are the two main sources of normative influence on students’ bullying behavior. It has been suggested that bullying is less frequent in classes where classmates hold more negative attitudes toward bullying (i.e., the injunctive anti-bullying class norm) and display more defending behaviors (i.e., the descriptive defending class norm). It has also been suggested that teachers’ responses to bullying incidents influence their prevalence within the classroom. However, most available studies have been cross-sectional, focused either on class norms or teachers’ responses, and provided inconsistent findings overall. The present study aimed to examine the influence of the injunctive anti-bullying class norm and the descriptive defending class norm and both teacher- and student-reported teachers’ responses to students’ bullying behaviors over time, in 1626 primary students and their 79 teachers. Multilevel analyses showed that the injunctive anti-bullying class norm and some of the teachers’ responses (i.e., working with the bullies and the victim, and disciplinary methods) predicted change in bullying over 5 months. This suggests that classmates and teachers have independent effects on bullying behaviors. The descriptive defending class norm did not predict bullying, which is in line with recent literature questioning the effectiveness of promoting defending behaviors among classmates. Our results suggest that anti-bullying programs should target both classmates’ attitudes and teachers’ responses to maximize effectiveness.</p>