Using rich longitudinal data from the Early Childhood Longitudinal Study-Kindergarten cohort (ECLS-K), we find that children who attended preschool enter public schools with higher levels of academic skills than their peers who experienced other types of child care (effect size of 0.14). This study considers the circumstances under which the preschool advantage persists, that is, the types of classrooms in which students who did not attend preschool "catch up" to their counterparts who did. Specifically, we focus on two dimensions of the early school environment-class size and the level of academic instruction provided. The findings suggest that most of the preschool-related gap in academic skills at school entry is quickly eliminated for children placed in small classrooms and classrooms providing high levels of reading instruction. Conversely, the initial disparities persist for children experiencing large classes and lower levels of reading instruction. These results point out that the longer-term effects of early childhood experience partly depend on classroom experiences during at least the first years of school. (Contains 2 figures and 8 tables.)