Pre-print, Working paper: Stereotypes, role models played by teachers and social norms are known to push girls to choose humanities rather than science. Do professors directly contribute to this strong selection by discriminating more against girls in more scientific subjects? Using the entrance exam of a French higher education institution (the Ecole Normale Supérieure) as a natural experiment, we show the opposite: discrimination goes in favor of females in more male-connoted subjects (e.g. math, philosophy) and in favor of males in more female-connoted subjects (e.g. literature, biology), inducing a rebalancing of sex ratios between science and humanities majors. We identify discrimination by systematic differences in students' scores between oral tests (non-blind toward gender) and anonymous written tests (blind toward gender). By making comparisons of these oral/written scores differences between different subjects for a given student, we are able to control both for a student's ability in each subject and for her overall ability at oral exams. The mechanisms likely to drive this positive discrimination toward the minority gender are also discussed.