I sit a lot during beginner classes. I also stand up from time to time. My beginner classes are smaller and pretty peaceful, it works for them. Except one of them...I have to stand to keep them focused. It is a beginner brass class, all boys, last period of the day.

I understand your question, but the real question is "are the kids engaged?" Maybe ask, "is the director missing things like bad posture or fingerings?" If the answers are yes and no respectively, then there's probably not a problem.