School suspension and expulsion are important forms of punishment that disproportionately affect Black students. Punishment early in schooling has consequences for long term achievement, criminal justice interaction, and wellbeing. Prior research identifies three mechanisms that help account for racial disparities in suspension and expulsion: between-school sorting, differences in student behaviors, and differential treatment of students with the same behaviors. We build upon and extend prior research in four ways: (1) by comparing the relative importance of the three mechanisms in a single study, (2) by focusing on students early in elementary school, (3) by measuring students' behavior at the time they enter school, before they have been exposed to school disciplinary practices, and (4) by using both teacher and parent reports of student behaviors. Using data from the Fragile Families Study and decomposition techniques, we find that between-school sorting accounts for 13% of the Black/White gap in suspension/expulsion, differences in behavior account for 9% of the gap, and differential treatment accounts for 39% of the gap. Behavior differences measured at age 9, which are likely to be endogenous to suspension and expulsion, reinforce the importance of differential punishment. Theoretically, our findings highlight differential treatment as a mechanism of early criminalization.