Achieving 90% of not is simpler than you think: ban smartphones from schools during school bours. If the parents want the kid to have a phone, then the parents can get a flip phone.
I commuted one hour from and to middle school and high school, I definitely needed a phone to communicate with my mother when stuff happened (and occasionally happened), such as missing the bus or being late for lunch.
You're neglecting emergencies that are happening in the school itself. School shooters, for instance.
I frankly don't see the problem with kids having the phone with them as long as they're not actually using it outside of an actual emergency.
And how is it enforced exactly? Are parents held responsible, or are state education funds impacted somehow based on smartphone use?
Would we need a federal mandate to require flip phone / feature phone support? The last time I tried to find a feature phone it wasn't easy, many depend on 2G/3G networks which are losing support and carriers have absolutely no incentive to carry feature phones when smartphones are all that sell.
A quick look at Indiana's law and the news articles are interesting. The law requires schools to implements rules that ban phone use during class, but the actual rules and implementations are left for schools to decide. The articles I found make that sound like the law is toothless and passes the hard work off to school systems, but in my opinion that's a great law as it let's every school do what works best for them without prescribing a single solution for everyone.