Starting from the observation that teachers treat boys and girls differently, this study investigated the variety in the magnitude of the difference in teacher-perceived teachability of boys and girls and its determinants. The study responded two research questions: (1) Does the extent of the difference in teacher-perceived teachability of boys and girls vary among teachers? (2) Do the student sex-composition of the school and teachers' gender role attitudes determine the extent of the difference in teacher-perceived teachability of boys and girls? The analysis of 1247 teachers in 59 secondary schools in Flanders revealed that a minority of the teachers do not notice a difference between boys' and girls' teachability, while a small group perceives boys to be more teachable than girls. The majority of the teachers perceive girls as more teachable than boys to a varying extent. A multilevel analysis showed an impact of the sex-composition of the school on the difference in teacher-perceived boys' and girls' teachability and an unanticipated interaction between the sex-composition of the school and teachers' traditional gender role attitudes: the more boys there are in school the larger the difference in teacher-perceived teachability of boys and girls, and this association is weaker for teachers with traditional gender role attitudes. Math/science teachers perceive girls' and boys' teachability less differently than teachers teaching other subjects. The study offers evidence for the importance of gender stereotypes as well as for the accuracy of teachers' perceptions.