The present study examines consequences of high peer pressure on academic performance in mathematics among 14-year-old middle-school students in Germany, Canada, and Israel. Its core assumption is that students with very high abilities in mathematics underachieve in order to avoid social exclusion (in the form of being called, e.g., nerds) in their schools. Altogether almost 1,700 students were surveyed in the three cultures. Findings of the study make it obvious that—concerning achievement in mathematics—the primary victims of peer pressure in middle school are girls, boys more likely than girls being the perpetrators. High individual preferences of achievement values can buffer effects of peer pressure. However, from a pedagogical point of view it may nevertheless seem an option to teach mathematics in mono-gender courses, in order to improve the chances of both girls and boys to show an academic performance that concurs with their real competence.