I have pupils clustered within schools, with a binary outcome (pupil experiences bullying or does not). So far all predictors are pupil-level data, and I run a the same model (same predictors) on each year of data. My teacher was talking about adding a school-level predictor: the size of the school, which will simply be the count of pupils in each school, that is the number of observations in each school. Is there any problem with doing this ? I tried to look for some references on this but I didn't find anything useful so far, and looking in some textbooks it doesn't seem like a normal thing to do ?
Edit: I am interested in how the residual variance at the school level changes over time.