Examined middle school disciplinary data in one urban school regarding three common hypotheses about disproportionate discipline based on gender, race, and socioeconomic status. Racial and gender disparities in office referrals, suspensions, and expulsions were somewhat more robust than socioeconomic differences. Boys engaged more frequently in a broad range of disruptive behavior than girls. Racial disproportionality in school suspension originated primarily at the classroom level. (Contains references.) (SM)