Links have often been drawn between individual personality differences among children, as captured by the Big Five personality traits, and academic achievement. Published research shows that conscientiousness is the most decisive factor, and its predictive power is greater when it is other- rather than self-assessed. In the current study (N = 161; Mage = 10.79 years), we examined the associations between both forms of personality assessment and two dimensions of school adjustment. Results confirmed that other-assessments are more statistically predictive of academic adjustment, but showed that self-assessments are more statistically predictive of subjective wellbeing at school. Complementary analyses in the form of generalized additive models indicated that the relationship between personality and wellbeing can follow either a linear or a nonlinear pattern, depending on which particular trait is considered. Such results may have practical implications for the choice of assessment method, and theoretical ones for the study of relations between personality and subjective wellbeing at school an area that is still relatively unexplored.