As a parent I'd rather have my child not districted by a phone in an emergency. My child will be safest in those situations if the staff contact the authorities and the kids are focused on following their instructions. In both situations, phone or no phone, there's nothing I can do until the situation is over.

Edit: and using the threat of school shootings yo ruin school for most children when so few schools will ever be in that situation is absurd. Those parents should put more of their energy into gun control and thr availability and affordability of mental health treatment.