Objective Researchers have demonstrated the prediction of academic functioning by children's prosocial behavior (PB). The goal of our study was to examine the contribution of adolescents' PB for middle and senior high school grades after controlling for stability of achievement and for intelligence, Big Five traits, and sociodemographic variables (i.e., sex and socioeconomic status). Method Study 1 examined on 165 adolescents (48.5% boys) the prediction by peer‐reported PB in 7th grade of academic achievement at the end of junior high school, after controlling for the above variables. Study 2 examined the prediction by 927 (52% girls) 8th graders' PB of academic achievement 5 years later, at the end of senior high school, taking into account the stability of grades, personality traits, and socio‐structural variables. Results Overall, hierarchical regression analysis indicated in both studies PB and Openness significantly predicted better grades in the short term and over time despite the high stability of grades across 5 years. Extraversion negatively predicted academic achievement across 1 year in junior high school. Conclusion Findings supported the view of PB as a strength and a key resource for adolescents' academic attainment.