AFAIK they do, but it is extremely challenging for teachers. Phone use can be very discrete, "aggressively monitoring phone usage" just isn't practical. And when teachers do confiscate phones the backlash against them is frequently strong and fierce. They face hyperbolic rhetoric about such confiscations being dangerous because "what if somebody needs to call 911!?" Even when administrators don't fold to the pressure, the teachers still need to put up with deranged rants and accusations from the parents. It's much easier for teachers if they pretend they didn't see the phone.

And schools can hardly screen every kid entering the school for a phone, every day. It would be invasive, slow, and expensive. Imagine having to go to school an hour early every day because you need to stand in line for TSA before entering the building. Not practical. We can't even keep cellphones out of prisons where invasive cavity searches are permitted. Jamming cell signals on school grounds is out of the question, it isn't legal. So given all these problems, why not sue tech companies for making these systems and services available on school grounds during school hours? I think it probably won't work, but it's worth a shot.