In recent times, many scholars have highlighted the emergence of group-based polarisation within Western societies. Research has demonstrated that this phenomenon has also reached educational settings, where groups of adolescents are opposed to each other. Surprisingly little has been written, though, on group-based polarisation in the classroom. This study examines along which lines (e.g. ethnic, socio-economic, or religious), how often and under which conditions group polarisation manifests itself in the context of secondary school classrooms. It does so by theorising this phenomenon, hypothesising its most common manifestation as well as the factors that may favour group-based polarisation. Following an exploratory sequential mixed-methods research design, consisting of a qualitative pre-study (N = 31) and an original survey that was fielded among Dutch secondary school teachers (N = 1034), it then demonstrates that: group polarisation, as perceived by teachers, is relatively rare in Dutch secondary education, generally occurring once or a few times per year; that it manifests itself predominantly along ethnic lines, and that the ethnic composition of educational contexts, tracking, and different educational stages statistically predict this form of perceived group polarisation. Based on our findings we propose that teacher training, focusing on how to cope with group-based polarisation in the classroom should be context-specific and tailor-made.