Gender segregation in employment may be explained by women's reluctance to choose technical occupations. However, the foundations for career choices are laid much earlier. Educational experts claim that female students are doing better in math and science and are more likely to choose these subjects if they are in single-sex classes. One possible explanation is that coeducational settings reinforce gender stereotypes. In this paper, we identify the causal impact of the gender composition in coeducational classes on the choice of school type for female students. Using natural variation in the gender composition of adjacent cohorts within schools, we show that girls are less likely to choose a traditionally female dominated school type and more likely to choose a male dominated school type at the age of 14 if they were exposed to a higher share of girls in previous grades. (Contains 13 tables and 6 figures.)