This is my second year stage managing at my high school, and I'm a senior. I've found that it's easier to get the younger kids to be quiet, but ones my own age or, even worse, my close friends, do not listen to me AT ALL. They don't see me as a stage manager or director (I do both), and to them I'm just a kid yelling at them. It even got to the point that I didn't talk to any of them for days because they were not respecting me or our advisor at all during rehearsal. I love my friends, I do, but sometimes they drive mr insane
Normally it takes our advisor screaming to get them to be quiet, and she hates to do that.
I've found that the best way to keep chatter to a minimum is to provide breaks. By giving the kids a chance to cool off during a break, it's easier to lead them when we get back to work.
But, it's high school drama. Try doing theatre at an amatuer or community theatre company- the difference is startling! When you work with actors who take the art form as seriously as you do, it's a great feeling!