The roles of Conscientiousness and Imagination in explaining sex differences in school performance were examined in two Flemish samples of school children using parental and teacher ratings of school performance (N = 599) and school grades (N = 448). Both personality domains predicted parental ratings of school performance and grades. In one sample, girls received slightly higher parental ratings of language achievement and overall performance ratings by teachers. However, controlling for Conscientiousness and Imagination facets, boys scored slightly higher for math and history. In this sample, lower externalising behaviour partially mediated the relation between Conscientiousness facets and school performance in girls but not in boys, but this pattern was not replicated in the second sample. We concluded that sex differences in school performance were small and many could be accounted for by personality traits. In some cases, however, personality traits acted to amplify sex differences in school performance. Copyright © 2008 John Wiley & Sons, Ltd.