While much research has focused on how Western schools contain or silence the increasing multilingualism of their pupils, this paper investigates how a Dutch-medium school in Brussels has decided to take a different approach by branding itself as multilingual. Based on sociolinguistic-ethnographic fieldwork, it will show that teachers invested in a multilingual school policy and that they recruited, and allowed pupils to speak, other languages for didactical purposes, as well as in more informal conversations. Nevertheless, as its curriculum remained predominantly Dutch-medium, the school was a site for contradictory behaviour: teachers problematised pupils' flexible language practices and limited proficiency in Dutch, and restricted their use of other languages out of a concern with pupils' acquisition of Dutch, access to curricular knowledge, and future educational and professional success. So, despite the school's attempts to transcend the struggles that arise in schools which are more averse to multilingualism, similar tensions emerge in this setting, as teachers need to find a balance between their pedagogical goals and concerns about monolingualism in the wider society.