In this ethnographic study, I present a single school's practice of registering and analysing absence from school. I show that teachers use various "dirty," interpretational contexts for understanding absence and make it classifiable in "clean" attendance categories -- a move that decontextualises the meaning of absence. When others in turn handle absence using such "clean," decontextualised data, they treat the many various absences as a single, compressed "absence pattern," a pattern which is explained in terms of a single reason derived from recontextualising the absence. I show that this process effectively excludes parents' and youths' own ways of interpreting absence. I argue from this that research should be mindful of the political nature of absence data, and that negotiating and gaining familiarity with children is a positive contribution to understanding absence and not unwanted dirt.