From the field of developmental psycholinguistics and from conceptual development theory there is evidence that excessive linguistic "code-switching" in early school education may pose some hazards for the learning of young multilingual children. In this article the author addresses the issue, invoking post-Piagetian and neo-Vygotskian ideas on children's cognitive development and its interplay with language in an argument for a linguistically "stable" pedagogy that prepares learners for the world of written language in which they have to express most of their learning in school. She argues that language development, literacy learning, and the development of concepts are intertwined phenomena in a child's development and that semantically and semiotically unambiguous and systematic classroom communication is therefore vital for learning success in the early grades when the foundations for cognitive academic language proficiency are laid. She calls for longitudinal research of young children's learning, investigating how language hybrids in classroom communication may impact it. Taking cognisance of linguistics theory that posits language as fluid, she hypothesizes that if language use, in mixed code, is excessive, it may lead to cognitive instability and insecure academic discourse. (Contains 2 figures and 3 endnotes.)