This paper focuses on the lack of impact on language education of recent paradigm shifts in the study of language and society such as the recognition of the ideology of language[s] as stable, discrete or bounded entities and the reality of heteroglossic languaging and semiotic practices in everyday life. Using South Africa as a case, the paper explores the implications of heteroglossic conceptualising of language as social practice for language education through three ethnographically informed case studies of classroom discourse. I will argue that monoglossic orientations which ironically underpin both monolingual and "multilingual" approaches have wide-ranging constraining effects on how children are positioned in schooling, and on children's participation in classrooms, resulting in a form of "epistemic injustice" (Fricker, 2007). [This article was written with Hannah Carrim, Alex Marshall, and Laura Layton.]