I don't know if it's necessary for the teacher to always say 'Please,' but I might expect some other way to soften it or make it a little funny, so as not to make the command sound too brusque.
Okay, everyone, we're going to do the next part standing up. Up, up, on your feet!
Now we're going to stand up. Come on, everyone out of their chair!
Let's stand up for this part, okay? Yes, really, come on, on your feet! Don't be a wimp, you need to move around or you might fall asleep!
In that vein, 'Please rise' or 'All rise' might be used as a humorous variant, but otherwise it's usually more formal, used for an entire audience at an event. (The kind where Britons might say 'Be upstanding,' causing Americans to giggle.) But even there it would be more formal and traditional; more modern would be just 'Please stand (for the national anthem).'