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 may be more prone to misbehavior as they get older, they may 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.