This article proposes an unusual identification strategy to estimate the effects of disruptive students on peer behavior and academic outcomes. Because boys with names most commonly given to girls 5 be more prone to misbehavior as they get older, they 5 become differentially disruptive in school. In elementary school there is no relationship between names and boys' behavior, but on transition to middle school, a large gap emerges in behavior between boys with names associated with girls and other boys. Using boys' names as an instrumental variable, I utilize data on names, classroom assignment, behavior problems, and student test scores from a large Florida school district in the school years spanning 1996–97 through 1999–2000 to directly measure the effects of classroom disruption on peer performance. I find that behavior problems are associated with increased peer disciplinary problems and reduced peer test scores, indicating that disruptive behavior of students has negative ramifications for their peers.