Using surveys with Danish students transitioning to secondary education, we study mental models of how gender and parental education shape academic performance. Students hold heterogeneous beliefs about performance gaps by gender and parental background, which appear to be shaped by within-family transmission and broader social environments. Open-text responses reveal that respondents link strong performance by girls and less socioeconomically privileged students to effort, while attributing privileged students' success to external advantages. Mental models matter: beliefs about performance gaps predict enrollment in upper secondary education by gender and parental education and causally affect students’ self-assessments, intended effort, and educational aspirations, as shown in an information experiment with girls. We highlight two mechanisms: updating about the returns to effort and about gender-specific effort costs in response to observed gender performance gaps. Our findings advance the understanding of education choices and shed light on the determinants and effects of mental models in a high-stakes setting.