Exactly, I could understand the principal stopping them from giving verses to other students who are compelled by law to be in school. No penalty would be appropriate but I could understand if the principal chose to stop them.
Teachers are adults, they are pretty much fair game for anything you want to talk to them about. They are not compelled by the state to be there. They freely took their job.
To follow your logic. If I'm driving my car, I am compelled by the law to observe the speed limit, drive responsible, etc., and others are compelled to do the same. Because of those compulsions, I am forbidden to have a christian bumper sticker or fish symbol anywhere on my car.