I study whether exposure to teachers' stereotypes, as measured by the Gender-Science Implicit Association Test, affects student achievement. I provide evidence that the gender gap in math performance substantially increases when students are assigned to teachers with stronger gender stereotypes. Teachers' stereotypes induce girls to underperform in math and self-select into less demanding high-schools, following the track recommendation of their teachers. These effects are at least partially driven by a lower self-confidence on own math ability of girls exposed to gender biased teachers. The findings are consistent with the hypothesis that stereotypes impair the test performance of ability-stigmatized groups, who end up failing to achieve their full potential.