Research has shown pervasive gender differences in academic achievement. Because both genders present similar global intellectual ability levels, interest has arisen concerning the role of non-intellectual factors in explaining these differences. In this study, the mediating role of personality dimensions related to disconstraint and aggressiveness, in the relationship between gender and academic achievement is assessed. The sample consists of 351 adolescents, ranging in age from 14 to 18 years old. The instruments are the aggressiveness and disconstraint PSY-5 dimensions of the MMPI-A and the School Life Survey (SLS). MANOVA shows that females report better indicators of achievement, the frequency of behavioral problems and overall satisfaction. Regression analyses using bootstrapping procedures reveal the full mediation effects of disconstraint, but not of aggressiveness, on the influence of gender on achievement. The results are discussed in light of the role of disconstraint-related personality characteristics for academic success and the potential factors underlying gender variability in these characteristics.