The goal of this study was to examine how different types of social experiences in kindergarten relate to Grade 1 academic achievement, while controlling for possible genetic and shared environmental influences through the use of the monozygotic (MZ) twin difference method. Social experiences in kindergarten included relationship quality with the larger peer group (i.e., rejection and victimization), relationship quality with one's best friend, and relationship quality with the teacher. Control variables included parental hostility-coercion, child cognitive skills and externalizing problems, and equivalent social experiences in Grade 1. Participants consisted of 223 MZ twin pairs ages 6 years at Time 1 (T1) and 7 years at Time 2 (T2). Results showed that within-pair differences in peer rejection and in poor teacher-child relationship quality at T1 uniquely predicted differences in MZ twins' academic achievement at T2. Mechanisms that could account for the possible causal role of these social experiences in regard to children's school achievement are discussed. (Contains 2 figures and 6 tables.)