To sort pupils at the end of primary school, some early-tracking systems apply a mechanism that unwittingly divides classes into two groups: students preparing for exams to enter better schools and everyone else, who decide not to compete for selective schools. Utilizing TIMSS data and a follow-up study in the Czech Republic, we show that this environment has a detrimental effect on pupils' self-confidence in mathematics, particularly among girls, who do not apply for selective schools but have peers in their classroom who do apply. Our results imply that gender gaps in self-confidence can result from school competitive environments.