Kindergarten is unique with respect to U.S. education policy. At no other time in their academic lives do children with such different prior-year educational experiences come together to form a classroom community, where they will learn with and from their peers. Yet, despite an assumption by parents and educators that peers matter, the relationship between the baseline achievement level of a child’s kindergarten classmates and the child’s learning during kindergarten is not well understood. Prior studies of early childhood peer effects have focused mostly on racial, gender, and socioeconomic classroom compositions, not on malleable, endogenous factors like academic skills. This study uses the national Early Childhood Longitudinal Study-Kindergarten Cohort 2010-2011 (ECLS-K:2011) to examine the association between classmate reading/mathematics scores at school entry and individual reading/mathematics learning in kindergarten. It is the first study to do so. Multilevel multivariate regression and a rich collection of individual, family, classroom, and school control variables are used to surface peer influence for reading and mathematics, and to determine if peer influence varies by baseline achievement level. The results indicate that class baseline skills are significant predictors of kindergarten year learning for both reading and mathematics. For reading, students with relatively low baseline skills appear more sensitive to changes in the class mean compared to students with higher baseline skills. For mathematics, there was no difference in sensitivity to peer effects between higher- and lower-achieving students. The findings indicate that peer skill level is an important factor associated with academic achievement in kindergarten. In fact, it appears to be more predictive of academic growth than other commonly used indicators of classroom quality. This study also revealed differences in the ways peer effects operate for reading and mathematics during kindergarten. These results call for researchers and policymakers to further investigate how peers contribute to the quality of the classroom environment, why observed subject matter differences may exist, and how interventions can create more equitable distributions of students within communities, schools, and classrooms.