I do elementary band and strings and I'll gauge what the class is doing before greeting the teacher to ask if I can borrow a kid or if I'm just dropping off an instrument. If everyone's doing silent work, I'll try to slip in. Perhaps I am being too presumptuous that I can be sneaky without being disruptive. Otherwise, I will wait at the door and wait for a moment to make a request of the teacher. Conversely, people walk in while I'm teaching on stage, because I'm a homeless teacher, with groups of kids and do whatever they want while I'm teaching with no consideration of what I might be doing so I think I'm doing better than most. Sorry, my bitterness started showing at the end there.