The present study examined the relations between two aspects of peer experiences in preschool classrooms and children's math and literacy development during kindergarten transition. Based on the theoretical framework of classroom ecology model, peer experiences were represented by classroom social network and teachers' grouping preferences. The outcomes included standardized math and literacy assessments collected from three time-points, fall of preschool, spring of preschool and fall of kindergarten. The analytical sample included 367 children recruited from 47 preschool classrooms located in two large school districts. Children completed peer-nomination tasks to characterize the classroom social network and teachers completed questionnaires to describe their grouping preferences. We conducted a multilevel longitudinal model to estimate the effects of classroom-level predictors on the trajectories of skill growth. We found that indices of classroom social network – density and centralization – were significantly associated with math and literacy development: more dense classrooms positively predicted math and literacy development whereas more centralized classrooms negatively predicted math and literacy development. Teachers' attention to children's skill levels in creating grouping structure was positively associated with math and literacy development. These findings indicate that young children's peer experiences are potentially important indicators of classroom quality, as they are associated with children's academic development over the course of preschool year into kindergarten transition. (author abstract)