Background: Schools around the world are increasingly diverse in ethnicity. Given the importance of peer acceptance in children’s well-being and development, it is a pressing concern for educators to promote intergroup acceptance in schools. Aims: First, to examine the developmental trends of acceptance of outgroup members in both the ethnic minority and majority students. Second, to investigate how outgroup acceptance is subject to the interplay between perceived norm of prejudice, a risk factor in the macrosystem, and teacher support, a protective factor in the microsystem. Sample: The participants were 3,723 students (ethnic majority: 61%; boys: 51.9%; mean age: 13.7) at Grade 2, 5, 8, and 11 from 24 schools in Hong Kong. Methods: The students were asked to complete a battery of questionnaires regarding their willingness to accept outgroup members in schools, perceived societal norm of prejudice against ethnic minority group, and teacher support they received in schools. Results: It was found that ethnic minority students accepted outgroup members more than ethnic majority students accepted them. The difference was primarily driven by the low outgroup acceptance of ethnic majority students in Grades 2 and 5. The results of multi-level analysis revealed that outgroup acceptance was associated negatively with perceived norm of prejudice but positively with teacher support. The negative association between outgroup acceptance and perceived norm of prejudice was attenuated in schools with supportive culture. Conclusions: The results point to the need of early intervention and the importance of cultivating school culture with strong social emotional support. Copyright © 2020 The British Psychological Society.