Diversity approaches in school may affect students' interethnic relations but are often only assessed through students' perceptions. We related teacher-reported diversity approaches (i.e., assimilationism, multiculturalism, color-evasion, and intervening with discrimination) to ethnic majority and minority students' ethnic attitudes as well as to their experiences or perceptions of ethnic discrimination. We also explored students' perceptions of teacher approaches as hypothetical mediators of teacher effects on interethnic relations. We coupled survey data from 547 teachers (M_age = 39.02 years, 70% female) in 64 schools in Belgium with large-scale longitudinal survey data from their students, including 1287 Belgian majority students (M_age = 15.52, 51% female) and 696 Turkish- or Moroccan-origin minority students (M_age = 15.92, 58% female) enrolled in the same schools (Phalet et al., 2018). Longitudinal multilevel models revealed that over time, teacher-reported assimilationism predicted (even) more positive attitudes towards Belgian majority members, and multiculturalism predicted less highly positive attitudes towards Belgian majority members among Belgian majority students. Teacher-reported intervening with discrimination predicted more perceived discrimination of ethnic minority students over time among Belgian majority students. We did not find significant longitudinal effects of teachers' diversity approaches with Turkish- or Moroccan-origin minority students' ethnic attitudes, nor with their discrimination experiences or perceptions. We conclude that teachers' multiculturalism and anti-discrimination approaches reduced interethnic bias and raised awareness of discrimination among ethnic majority students. However, different perceptions by teachers and students suggest the need for schools to better communicate inclusive diversity approaches.