We're talking about kids, not adults. You ban cell phones for the same we weren't allowed to play our Gameboy during class when I was a kid. They lack the self control and decision making capabilities to forgo something fun for the sake of something important.

Not to mention we have plenty of studies that show even a silent phone sitting quietly in your pocket or on your desk can be an attention drain, as you're subconsciously waiting for a notification to go off.

I'm amazed it took this long for the schools to finally ban the damn things.