Black and poor students are suspended from U.S. schools at higher rates than white and non-poor students. While the existence of these disparities has been clear, the causes of the disparities have not. We use a novel dataset to examine how and where discipline disparities arise. By comparing the punishments given to black and white (or poor and non-poor) students who fight one another, we address a selection challenge that has kept prior studies from identifying discrimination in student discipline. We find that black and poor students are, in fact, punished more harshly than the students with whom they fight.