The purpose of this study is 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. The 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 findings also reveal 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.