This paper tests whether we observe sex-discrimination in teachers’ grades, and whether such biases affect pupils’ achievement during the school year. I use a unique dataset containing standardized tests, teachers’ attributed grades, and pupil’s behavior, all three at different periods in time. Based on double-differences, the identification of the gender bias in teachers’ grades suggests that (i) girls benefit from a substantive positive discrimination in math but not in French, (ii) girls’ better behavior than boys, and their initial lower achievement in math explain less than 10% of this gender bias. Second, I use the heterogeneity in teachers’ discriminatory behavior to show that classes in which teachers present a high degree of discrimination in favor of girls are also classes in which girls tend to progress more over the school year compared to boys.