This article aims to deepen our understanding of the dynamic interaction between language policies, school characteristics and teachers' beliefs about monolingualism. The study takes place in Flanders (Belgium), a region characterized by educational policies which are based on a stringent monolingual ideology. Based on a survey of 775 teachers from across 48 secondary schools, we examined how these policies affected teachers' beliefs, whether and how teacher beliefs vary between schools, and what the consequences of these beliefs are. The results of our multilevel analysis indicate that teachers strongly adhere to monolingual policies, while there are also significant differences across schools, which are related to the ethnic composition of those schools. Furthermore, a stronger adherence to monolingualism was found to trigger teachers to have lower expectations about their students but not about their ability to teach. Finally, implications for policymakers are discussed.