At many universities, English serves as a lingua franca (ELF) between teachers and students for whom English is not their L1. Despite the spread of English-medium instruction (EMI), empirical research on the nature of teacher-student interactions in EMI classrooms remains limited. This study examines the use of ELF in EMI engineering classes at a university in Turkey to explore how teachers and students use code-switching as a communicative strategy in classroom interactions. Data were collected and analysed using a qualitative approach. Nearly 14 hours of classroom observation data were collected from three lecturers and analysed according to patterns of classroom interaction and language use. The findings suggest that teachers and students prioritize communicative efficiency over an adherence to monolingual, NS norms in classroom interactions. Pedagogical implications are discussed for ELT specialists tasked with preparing students for academic study in English and supporting content lecturers in EMI settings.