The negative consequences of perceived ethnic discrimination on adolescent adjustment are well documented. Less is known, however, about the consequences of discriminatory climates in school, beyond the individual experiences of discrimination. This study investigated whether a perceived discriminatory climate in school is associated with lower academic performance across adolescents from ethnic minority and majority groups, and which psychological mechanisms may account for this link. Using the 2018 Programme for International Student Assessment (PISA) data, the participants were 445,534 adolescents (aged 15–16, 50% girls) in 16,002 schools across 60 countries. In almost all countries, a discriminatory climate—i.e., student perceptions of teachers’ discriminatory beliefs and behaviors in school—was associated with lower math and reading scores across all pupils, although minorities perceived a more discriminatory climate. Lower school belonging and lower values attributed to learning partially mediated these associations. The findings demonstrate that schools’ ethnic and racial climates predict standardized academic performance across schools and countries among pupils from both ethnic majority and minority groups.