Given the plague of guns and school shooters, parents and kids may want phones in the classrooms.We adults forget that kids these days are like a hive, and see that as a feature, not a bug.
Even if the phones won’t protect the kids, at least they can share their last moments with each other and/or their parents. 
It’s against federal law to block cellphone signals, which is easily done.  But you can use enough copper in a building’s design to make it into a Faraday Cage, which does the trick.
Dave Porter
Minneapolis