My modeling experiences:
High school art classes - absolutely no nudity, costumes only (the teacher was a friend of mine, so I modeled for his classes as a favor).
College and private art classes - both costumed and nude. Had only one problem with a high school student who had brought an obnoxious/immature friend to class (Steve didn’t drive, so his friend brought him - I was okay with the non-artist in the room, but he proved he couldn’t control his commentary).