It is consistently reported that despite equal cognitive ability, girls outperform boys in school. In several methodological steps, the present study examined sex differences in school achievement and some of the most important personality and motivational constructs in a sample of 204 females and 138 adolescent males (mean age M = 16.94 years; SD = 0.71). Grades in Math and German as well as grade point average (GPA) served as achievement criteria. Intelligence, the Big Five of personality and motivational variables (achievement motives, goal orientation, task values and ability self-concepts) served as predictors. After controlling for intelligence, girls' grades were significantly better than boys'. Mean sex differences were found for most variables. There were no gender-specific associations between predictors and grades. Agreeableness, work avoidance, ability self-concepts and values ascribed to German mediated the association between sex and grades in German. Controlling for ability self-concepts and values ascribed to Math enhanced the association between sex and math grades. We concluded that personality and motivation play important roles in explaining sex differences in school attainment. Results are discussed against the background of practical and methodological implications. Copyright © 2008 John Wiley & Sons, Ltd.