Language competence is both the means and the end to educational achievement, and multilingualism in particular has important cognitive, academic and societal advantages. The linguistic diversity in South Africa creates an ideal context to provide learners with the educational opportunities that promote high levels of linguistic proficiency in their home and additional languages. Unfortunately, the education system has not delivered on the constitutional imperatives of promoting multilingualism. English continues to dominate as the preferred language of teaching and learning, at the expense and marginalisation of the African languages. This is regarded by many researchers as the primary reason for the disturbingly low numeracy and literacy achievement levels of the majority of South African schoolchildren. However, the effects of language-in-education practices on academic achievement are not straightforward. This paper analyses recent research pertaining to the effects of language-in-education practices and argues that the critical role of educational linguistics is neglected in the South African education system. This affects the quality of teaching irrespective of the language of instruction and has a significant impact on the achievement of our children. The purpose of this paper is to present a critical theoretical perspective on language in education in order to influence policy and practice. An additional aim is to promote the role of speech-language therapists (SLTs) in education, since these professionals are well positioned to work in collaboration with educators to enhance language learning in mainstream/ordinary classrooms. However, SLTs also need to be well informed about the challenges in education and the theory underlying language-in-education practices.