The dynamics of the construction and perpetuation of gender stereotypes are classic research subjects in social psychology and in the field of educational guidelines in France. The most recent government decree aims to counter stereotypes in schools, by exposing pupils to counter-stereotypes. This study examines the effects of activating counter-stereotypes, versus stereotypes and gender neutral input, on the test performance of female and male fourth-grade students in a mathematical task in a "real-life" classroom setting. Theoretical implications of social mechanisms to combat gender stereotypes are discussed.