This study assesses the effect of secondary schools' sex-composition on boys' and girls' felt pressure for gender-conformity. Whereas gender-conformity pressure affects adolescents' well-being, little large-scale research examines school features' influence. Multilevel-analyses are conducted on representative Flemish data of first grade students, 2688 boys and 2430 girls, in 57 secondary schools, gathered at the beginning of school-year 2012/13 and the end of 2013/14. When estimating the effects on gender-conformity pressure at time 2, the gender-conformity pressure at time 1 is accounted for. For girls, no effects of the school's sex-composition are found. Boys in schools with more girls display lower levels of felt pressure at the start of secondary education. The felt pressure at time 2 increases with the proportion of girls, indicating that boys forming a numerical minority at school, report more growth in gender-conformity pressure. This effect is stronger for boys holding more traditional gender-role attitudes.