The chapter addresses the need to move beyond the application of ‘northern’ theoretical models of language and pedagogy in policy and research in Sub Saharan Africa through the discussion of findings from an ethnographic case study of mathematics lessons in a government secondary school in rural Rwanda. The unit of analysis for this study is ‘mathematics lessons’, which are understood as discursive, situated and socio-cultural activities. Data comprise 13 hours of audio and video recorded lessons, observed over 5 months, transcribed multilingually and descriptively; post-lesson interviews with the teacher and student focus groups; fieldnotes from participant observation at school; and photographs of the classroom and school environment, the coursebook, black board and student notebooks. Data were analysed inductively to identify patterns and hierarchical units of interaction, and situated and socio-cultural meanings. The chapter demonstrates that the teacher and students perceive the students’ language ability as lacking in relation to the monolingual ideal of the language in education policy which inhibits students’ confidence to communicate in lessons. The chapter finishes with recommendations for future language in education policy research and teacher development in Rwanda and comparable contexts.