Those studies speak about social media addiction, smartphone addiction, and excessive phone use and not about having a phone.

If you want to prevent the negative effects of social media and (designed to be) addictive apps, then you should ban social media and addictive apps, and not phones (because that would just mask the symptoms).

The schools here I have experience with are stricter about smart watches than phones - you can get everybody to turn their phone off all day or keep it in a locker, but it's much harder with watches.

I assume those effects are similar even if they're only using the phone before and after school.

Probably, but it would be much less reasonable for the state to enforce outside of school (both for practical and ideological reasons).

Probably not, and replacing checking it every 45 mins with a continuous 8 hours of no screen time matters a lot, and a ban is a solution alone.