By now, most schools in Denmark are banning phones during school hours. My kids' school did it two years ago. I have no idea if it has improved my kids' "cognitive skills", and frankly I don't care that much about their academic level. They are kids. They should run around, play and be happy, and then they will learn what they need.
As a parent it's wonderful to know that the kids have this 5-7 hour break from the screens. Just wonderful.
Someone realized it's not a good idea to hand a bunch of teens cameras, give them unlimited possibility to bully eachother anonymously and then force them to share a space for 8 hours every day, including changing clothes and showering for gym class. In hindsight it seems obvious.
I think that's the norm in UK secondaries too. My 11yo is allowed to take his phone to school for but policy is it stays switched off, in the locker, until the end of the school day.
As a parent it's wonderful to know that the kids have this 5-7 hour break from the screens. Just wonderful.