The way a class treats a substitute is what that class is really like.

Behavior changes when the usual consequences step out, and what appears in that gap is the actual state of things. A group under no supervision shows what it was performing before. That is uncomfortable to notice about a room you belong to, because you belong to it. The substitute is not the subject here; the room is.

Watch what you do when the person with authority is absent. Ask whether you would behave the same way in front of the usual one. If the answer is no, that difference is worth closing.