Judging by the amount of time my two (17 and 13) spend on their phones, children already have a very 'healthy' relationship with their phones. The teaching of awareness of online safety, mental well-being, cyber-bullying etc does not require the child to have a phone with them.
As a parent, I am glad to be certain that they get a solid 8 hours a day when they are unable to be glued to it at all, and where there is no risk of it being lost, damaged etc. Sure, it's occasionally annoying when they have after school activities but, in any real urgent need, the school will let them use an office phone.
And I say all this as a confirmed technophile. I have nothing against phones - I use mine probably far more than I should. But the struggle to get them to look away from it at home is palpable and constant.
It's debateable how sustainable the policy will be in the longer term though. At some point, I can see their bus pass moving from a smartcard to an app. Whilst not many people use buses to get to their school, that would probably be a turning point.