Cell phones should be labeled with the student's name and placed in a portable bag when entering the classroom. The students can reclaim their phones when the class is over. In the event of weather emergency, lock down, or whatever the teacher should be trained to take the bag with her as she ushers her students to shelter where the students can reclaim their phones to assure parents they are okay or whatever.
I have attended training seminars, spiritual retreats, etc. in which everybody was required to turn in their cell phones, wrist watches, any and all communication devices with the outside world unless medically necessary. It was always a bit traumatic not having those things for awhile, but after awhile you get used it and rather enjoy the liberty of being free of time restraints/expectations etc. I'm sure students would too.
Sometimes I wish underage persons were restricted to the old style mobile phones that ALL you could do with them is make or receive calls and wrist watches that did nothing but tell time. Then you would have kids that actually make eye contact and converse with people again and that have to do some intellectual work to learn the subject matter so they are actually educated.