In this study, we aimed to explore Grade 6 mathematics teachers' use of English, the language of learning and teaching in assessment for learning in selected primary schools in Alexandra Township, South Africa. From Grade 4, English is the language of teaching and learning for most learners, despite English being the home language of a minority of learners. Results of studies have shown that in South Africa, in Grades 1 to 3, in which learners are taught using their home-language performance appears to be better than in Grades 4 to 6 where English as a First Additional Language (EFAL) is used for teaching and learning. Guided by qualitative case study design, we used semi-structured interviews and non-participatory observation to collect data from nine purposefully sampled Grade 6 mathematics teachers. In conjunction with the literature reviewed and the theory underpinning the study, we used themes to analyse, interpret, and discuss the data we collected. This research revealed that learners in Grade 6 struggle to understand English as the language of learning and teaching, so, to augment concept development and understanding, teachers and learners use code-switching. In the classrooms observed, this practice has become the norm to improve the performance of learners with limited language proficiency. However, because of the differences between the home language of learners and that of teachers in mathematics classrooms, code-switching is often not enough to ensure understanding.