I'm just saying the danger is if he or she's in a jammed area and doesn't KNOW IT, that could be dangerous. If he or she knows which areas are jammed and is on call, he or she can avoid those areas for fear of missing an important call (and hiding in a jammed area to avoid calls could be grounds for dismissal).
That's all I'm saying. If everyone starts randomly jamming and nobody knows, that could cause real problems, even if it would eliminate a lot of annoyance. I think schools confiscating people's cell phones arbitrarily is also unethical (even if they want to claim the constitution doesn't exist on school grounds).