In most developed countries, students have to choose a major field of study during high school. This is an important decision because it largely determines subsequent educational and occupational choices. Using French data, this paper reveals that enrollment at a more selective high school, with higher-achieving peers, has no impact on boys, but a strong impact on girls’ choices: they turn away from scientific fields and settle for less competitive ones. Our results are not consistent with two commonly advanced explanations for gender differences in field of study—namely, disparities in prior academic preparation and in sensitivity to rank in class.