We present in this paper evidence about the effects and mechanisms of gender peer effects in elementary, middle, and high schools. For identification, we rely on idiosyncratic variations in gender composition across adjacent cohorts within the same schools. We find that an increase in the proportion of girls improves boys and girls’ cognitive outcomes. These academic gains are mediated through lower levels of classroom disruption and violence, improved inter-student and student-teacher relationships, and lessened teachers’ fatigue. We find no effect on individual behavior, which suggests that the positive effects of girls on classroom environment are mostly due to compositional change.