This article examines the association between school ethnic composition and immigrant students' intentions to finish high school and to move on to higher education. We used data from 1324 immigrant and 10,546 native students gathered in the school year 2004-2005 in a sample of 85 Flemish (Belgian) secondary schools. Logistic multilevel analyses (HLM6) show that students attending schools with a majority of native students (enrolling less than 20% immigrant students) were twice as likely to plan to finish high school and to plan for higher education than those attending high concentration schools (more than 50% immigrant students). These associations were due to students' socio-economic status (SES) and there was no difference in aspirations between high and low concentration schools after controlling for students' SES and the SES context of the school. All else being equal, immigrant students in high concentration schools tended to aspire to finish high school and move on to higher education slightly more than those attending medium concentration schools (20-50% immigrant students). The analyses further show that these differences between high and medium concentration schools can be explained by the more optimistic culture in high concentration schools. The main conclusion is that high concentration schools are not necessarily detrimental for students' educational aspirations. (Contains 3 tables and 2 notes.)