We only had one teacher tell us we were required to (we were standing up just kinda mumbling it), the following class everyone just stayed seated and at least 3 of us had printouts in our backpacks confirming that it was a violation of the first amendment to require it.
He didn't even acknowledge it, knowing that teacher I imagine he looked it up right after and realized he was wrong.
We eventually resumed standing up but hands at our sides and silent.
Edit: if this is showing up as a reply to another comment, it was supposed to be a top level comment to the OP. Some weird stuff keeps happening with my comments but I'm not sure if it's just visual.