Time-sampled observations of Head Start preschoolers' (N = 264; 51.5% boys; 76% Mexican American; M = 53.11 and SD = 6.15 months of age) peer play in the classroom were gathered during fall and spring semesters. One year later, kindergarten teachers rated these children's school competence. Latent growth models indicated that, on average, children's peer play was moderately frequent and increased over time during preschool. Children with higher initial levels or with higher slopes of peer play in Head Start had higher levels of kindergarten school competence. Results suggest that Head Start children's engagement with peers may foster development of skills that help their transition into formal schooling. These findings highlight the importance of peer play, and suggest that peer play in Head Start classrooms contributes to children's adaptation to the demands of formal schooling. (author abstract)