This study examined whether adolescent students' externalizing behavior and prosocial behavior affects their academic achievement (i.e., school grades and standardized test scores) in two major academic domains (i.e., mathematics and German) over and above well-established determinants of school achievement (i.e., prior achievement, intelligence, and domain-specific self-concepts). The study draws on longitudinal data from two independent samples of students (A/B) who were each followed from Grade 7 to Grade 9 (N[subscript A/B] = 1143/1345). In each academic domain, structural equation models showed that externalizing behavior negatively predicted and prosocial behavior positively predicted academic achievement. When both behaviors were included simultaneously, prosocial behavior particularly predicted school grades in both domains, whereas externalizing behavior particularly predicted test scores in mathematics. Further analyses did not suggest differences between boys and girls in the effects of social behavior on academic achievement. Implications for future research and educational practice are discussed.