This paper shows two different studies aimed at exploring whether student’s gender has an influence on the way teachers assess performances in mathematics. The first study, an experimental study, involved 48 mathematics teachers (higher secondary education). They were divided in two comparable groups and had to rate four assignments attributed in one group to a girl, in the other group to a boy. The findings show that even if teachers show the same expectations regarding boys and girls’ abilities and interest in mathematics (assessed by a questionnaire), a slight but significant gender bias is nevertheless at play when rating equivalent assignments attributed to boys or girls, depending whether the assignment is good or bad. The second study, a correlational one, is based on the PISA 2000 data. The PISA maths score have been correlated with the students’ marks in mathematics gathered through the background questionnaire. Results show that high achievers males are overestimated while high achievers female are underestimated. The reverse pattern is observed for low achievers: among those low achievers, males are underestimated and females overestimated.