Background: Research exploring mechanisms driving inequalities in school systems has found that biased teacher judgements contribute to observed disadvantages for ethnic minority students. Teacher judgements may be driven by explicit and implicit attitudes. Aims: The current research explored the effect of cultural diversity at schools (actual or imagined) on teachers' attitudes towards ethnic minority students. Samples: One hundred and five preservice teachers (90 female) with a mean age of 26.20 years (teaching experience: 57.55 weeks) participated in Study 1. Two hundred and thirty-one teachers (159 female) with a mean age of 41.00 years (teaching experience: 12.92 years) participated in Study 2. Method: Cultural diversity was operationalized via a fictive description of a school (Study 1) or via the actual proportion of ethnic minority students at the school (Study 2). An Implicit Association Test assessed implicit attitudes towards ethnic minority students. Explicit attitudes were assessed via questionnaire. Results: Preservice teachers imagining a more culturally diverse school held more negative implicit attitudes towards ethnic minority students than those imagining a less diverse school. In contrast, in-service teachers actually working in more diverse schools held less negative implicit attitudes towards minority students. Preservice teachers associated teaching in culturally diverse schools with increased effort, whereas in-service teachers actually working in culturally diverse schools reported more enthusiasm towards teaching ethnic minority students. Conclusions: This research shows the challenge and the negative stereotypes preservice teachers associate with culturally diverse schools, while in-service teachers' negative associations may be buffered by the actual experience of working with ethnic minority students.