This study attempted to untangle how two dimensions of school racial/ethnic composition--racial/ethnic diversity of the student body and racial/ethnic matching between children and their peers--were related to socioemotional and academic development after the transition into elementary school. Analysis of the Early Childhood Longitudinal Study-Kindergarten Cohort revealed that school racial/ethnic composition was more strongly associated with children's academic, as opposed to socioemotional, outcomes. Students had higher achievement test scores in more diverse schools, especially when they also had more same-race/ethnicity peers in these diverse schools. These patterns were particularly strong for White students. Having more school peers of the same race/ethnicity, regardless of the overall level of diversity in the school, was associated with positive socioemotional development. (Contains 3 tables, 1 figure, and 1 note.)