Seems like a straightforward solution to a simple problem. Pretty direct correlation with kids having access to smartphones vs declining grades and attention spans.
In schools that have started simply not allowing phones in class, grades have improved. This seems like the next logical step. Its basically just returning to pre-smartphone days. Schools used to confiscate phones if you had them out at all during school hours.
People will disagree with simple solutions, but those are often the ones that works. Phones cause problems, get rid of phones. Simple and direct.