In this paper, we make a theoretical and methodical case for combining Institutional Ethnography and Reflexive Grounded Theory to investigate linguistic transition processes of newly-arrived students in the German school system. Legitimised by a missing knowledge of German, the students are separated into preparatory classes in order to prepare them (linguistically) for the regular classes. This article develops a reflexive institutional-ethnographic approach that problematises and visualises the various voices and practices of social actors engaged in these transition processes. We elaborate the theoretical premises, methodical steps and procedures of a combined approach which allows to reconstruct the transition moment and its attendant tensions, stakeholder perspectives and embeddedness in wider social processes of migration-related multilingualism. The paper enriches the methodological landscape in language, education and migration studies and offers implications for engaging with educational research sites.