Background: In educational research on children's academic performance, few topics have received more attention than the consequences of school segregation and the impact of self-fulfilling prophecies. However, virtually no research has investigated whether self-fulfilling prophecies account for the impact of school composition on academic achievement. Purpose & Research Objectives: This study aims to integrate research on the effects of school segregation with that on self-fulfilling prophecies by examining the mediating role of teacher expectancies regarding the impact of school composition on pupils' math achievement. First, we investigate whether teachers' teachability expectations are related to the socioeconomic and ethnic composition of the school. Second, we investigate whether and how the effects of school composition can be explained by self-fulfilling prophecies. Because it is theorized that teacher expectancies might have an impact on pupils' academic achievement through pupils' perceptions of control over their achievement, we investigate the role of pupils' sense of academic futility. Sample & Research Design: Quantitative data from a survey of 2,845 pupils and 706 teachers in 68 Flemish (Belgian) primary schools and qualitative data obtained through in-depth interviews with 26 teachers in five schools are analyzed. A complementary mixed-method design is used: Findings from the quantitative data are strengthened and illustrated with qualitative data. Results: The multilevel analysis shows that teachers' teachability expectations are lower in schools with a high share of nonnative and working-class pupils and that these teachability expectations have an indirect impact on pupils' achievement through pupils' feelings of academic futility. The qualitative analysis reveals that the low teacher expectations in these schools are largely triggered by alleged linguistic deficiencies and problematic language use of the pupils and that school staff persistently communicate their preference for Dutch monolingualism to pupils. Recommendations: The results of this study indicate that socioeconomic desegregation may not be needed if it is possible to reform schools with a larger share of working-class pupils. Schools that produce more favorable teachability expectations are recommended. In particular, teachers' attitudes and beliefs regarding pupils' linguistic backgrounds might be the focus of educational reforms.