This paper examines the spillover effects of misbehaving boys and girls on others’ behavioral problems, leveraging the random assignment of students into classrooms in South Korean middle schools. Peers with single parents are more likely to misbehave in class, providing a proxy for peers’ misbehavior to overcome the reflection problem. By using the composite index of class misbehaviors, I find that boys with single parents lead to an increase in the intensity of other students’ misbehaviors in class, whereas girls with single parents have a negative but statistically insignificant effect on others.