In France, most teachers still receive scant training in how to support plurilingual children in their learning of and through the language of instruction. In the absence of relevant, in-depth knowledge about language, we believe that many teachers are practising language policies based on beliefs rooted in ideologies unsupported by research findings. In this paper, we will present and analyse data from interviews with head teachers ("n" = 46) from a variety of schools in the Strasbourg area, north-east France. Critical, interpretive analysis of the reported discourse reveals evidence of linguistic hierarchies, separate spaces for different languages, a profusion of bilingual myths and a persistent monolingual habitus at school. Our findings underline the importance of uncovering and analysing teachers' language ideologies in a bid to better understand the influences and obstacles preventing them from practising informed language policies with respect to plurilingual language development.