In an Equity situation, if an actor is in rehearsal, regardless of the rehearsal, I think a SM should be present. In the cast of a school situation like this, I still think the SM should be present - although not necessarily in the room with them, but just to make sure it is set up, running well, doors are unlocked, etc, etc.
I remember once when doing a dance show, I was not required to be at rehearsals, but I did attend a lot of them, to the point I was expected. One rehearsal I chose not to be there, I received a phone call asking where I was . . . no one had brought keys to the rehearsal space.
Little things like that come up; it's nice to be to handle them, but in the end there is no right or wrong answer for this - it's all a question of the style of the SM.