These policies already exist but they're practically unenforceable. Individual teachers are incentivized to ignore it because enforcing these rules causes trouble for them. And even the most enthusiastic enforcement will fall flat; discrete phone use is trivial. Even paranoid proctors can't keep phones out of classrooms during exams; asking a teacher to do such policing while also trying to teach a lesson isn't going to work. This is what they're already trying to do (or were trying to do, before giving up) and it isn't working.