To improve our understanding of where to target interventions, the study examined the extent to which school discipline disproportionality between African American and White students was attributable to racial disparities in teachers' discretionary versus nondiscretionary decisions. The sample consisted of office discipline referral (ODR) records for 1,154,686 students enrolled in 1,824 U.S. schools. Analyses compared the relative contributions of disproportionality in ODRs for subjectively and objectively defined behaviors to overall disproportionality, controlling for relevant school characteristics. Results showed that disproportionality in subjective ODRs explained the vast majority of variance in total disproportionality. These findings suggest that providing educators with strategies to neutralize the effects of implicit bias, which is known to influence discretionary decisions and interpretations of ambiguous behaviors, may be a promising avenue for achieving equity in school discipline.