Funded by the FNRS (PDR T.0091.16, 2016-2020) and the ULB/Brussels-Wallonia Federation (Action de recherche concertée, 2016-2019), this project sets out to describe and understand how teachers reconcile the irreconcilable: it explores how teachers in heteroglossic urban schools implement linguistic policies predicated on a monoglossic view of education, social mobility and society. In particular it analyses (1) how teachers articulate a monolingual policy; (2) if and to what extent teachers are responsive to linguistic diversity; and (3) which conditions facilitate these articulations and responses.