Objective: In multiethnic classrooms, acceptance and rejection by classmates of one’s own versus other ethnicity is influenced by in-group preference, the societal status of the ethnicities, and composition of classrooms. We aimed at (a) confirming these effects for immigrant versus nonimmigrant adolescents in newly formed classrooms, (b) longitudinally studying the change of these effects over the next 2 years, and (c) studying the longitudinal links between immigrants’ acculturation and acceptance/rejection by (non)immigrants. Method: This was a multilevel, longitudinal study of 1,057 13-year-old students nested in 49 classrooms over the first 3 years of middle school in Greece. Immigrant composition of classrooms varied strongly (average 44%), and immigrants in a classroom were ethnically homogeneous (78% same-ethnic). Students’ acceptance and rejection by Greek and immigrant students were sociometrically assessed every year. Multilevel analyses were conducted for questions a and b and cross-lagged analyses for question c. Results: Initially, immigrants were less accepted and more rejected by their classmates than Greeks. However, in classrooms with more than 66% immigrants, they were more accepted and less rejected. Over time, (a) immigrants and Greeks did not differ in being rejected and (b) immigrants in classrooms with few immigrants became increasingly more accepted. Finally, immigrants with higher involvement with the Greek culture were more accepted by their Greek classmates. Conclusion: Immigrants’ peer relations with Greeks were positively affected by increasing opportunity for intergroup contact and involvement with the Greek culture. Interventions supporting acculturation and intergroup contact may prove beneficial for immigrant students.