The present study was conducted to analyze whether in-class friends influence each other's grades, and whether adolescents tend to select friends that are similar to them in terms of academic achievement. During 1 academic year, 542 eighth-grade students (M age = 13.3 years) reported on 3 different occasions on their in-class friendship networks. At these occasions their report card grades for 3 subjects were copied from their files. We tested whether academic achievement functions as a selection criterion for friendship, and whether academic achievement is influenced by in-class friends, using social network analytic techniques. Socialization effects for Dutch and English language grades, but not for mathematics grades, were found. We found no support for selection effects of grades.