This paper applies Dennis' [(2009). "What does it Mean when an Ethnographer Intervenes?" "Ethnography and Education" 4 (2): 131-146] modes of ethnographic intervention to a fieldwork experience of an observed secondary school lesson in England. Ethnographic research raises numerous ethical dilemmas, in the face of which "intervention" is unavoidable. The observed lesson--in which a teacher was judged as "Requiring Improvement"--left me with "guilty knowledge". The performative nature of observed lessons constructs highly charged events. Drawing particular attention to the power imbalances between observer and observed, ethical deliberation about the event is considered, and subsequent "interpersonal" and "administrative" intervention is presented. As ethnographers, it is impossible to avoid intervening in some sense. I conclude that performativity raises ethical issues which may demand particular responses from ethnographic researchers, whose empathetic intention places them well to explore--and critically engage with--the workings and effects of performativity. [For "What Does It Mean when an Ethnographer Intervenes?," see EJ856724.]