This article examines what we call "micro-level language policy-in-process"--that is, how a target--language-only policy emerges "in situ" in the foreign language classroom. More precisely, we investigate the role of "language policing", the mechanism deployed by the teacher and/or pupils to (re-)establish the normatively prescribed target language as the "medium of classroom interaction" in the English as a foreign language classroom of an international school in Sweden. Using ethnomethodological conversation analysis, we have identified a regular three-step sequence for language policing: (1) a (perceived) breach of the target-language-only rule, (2) an act of language policing and (3) an orientation to the target-language-only rule, usually in the guise of medium switching to the target language. Focusing primarily on teacher-to-pupil policing, where the teacher polices pupils' (perceived) use of their L1 (Swedish), we identify three different categories of teacher-policing. These categories are based on particular configurations of features deployed in the three steps, such as initiator techniques (e.g. reminders, prompts, warnings and sanctions) and pupils' responses to being policed (e.g. compliance or contestation).