That’s the policy at most schools. Actually enforcing that in the face of a classroom of kids who don’t respect the rule? That’s a much bigger problem. They’re a lot more clever at sneaking them out than you would think. Moreover, if the phones are just feet from them, their presence is never out of mind. They’re a constant distraction even in a bag. Phone apps are literally designed to be addictive. Imagine if we had a rule that said “crack pipes are fine in your bag. As long as you don’t take them out and smoke in class, you’re fine.” Even if we lived in a world where crack somehow was legal for minors to have, how effective to you think that rule could be enforced?
Trust isn’t the issue. Probability is. Even without deception, there’s a chance someone can have an STD without knowing it. And there’s a chance that std won’t show up on testing due to incubation times, dormancy phases, and false negatives.
Imagine there is a 1% chance of your partner having an STD without knowing it. 1% doesn’t sound too bad an odds. But if you have 50 partners in an extended polycule, then the chance that at least one of them unknowingly has one is 1-(.99)^50, or 39%. Probabilities compound.