Much of the research on adolescent friendships and school achievement has focused on in-school friends, ignoring the potential effects of having out-of-school friendships. The goal of this study was to examine the relation between having relatively more in-school friends and school achievement among a sample of over 600 12th grade students from ethnically diverse backgrounds. We found that adolescents with more in-school friends, compared with out-of-school friends, had higher grade point averages (GPAs) and that adolescents with higher GPAs had more in-school friends. These relations were mediated by academic experiences, including those shared with friends. However, as hypothesized, the social aspects of adolescents' friendships did not vary according to their percent of in-school friends, attesting to the importance of considering both types of friendships in understanding adolescents' social experiences. None of the relations described varied according to gender or ethnicity.