Maybe this helps to get some background and scientific data on the discussion:
https://jonathanhaidt.com/anxious-generation/
TL;DR: The data shows that the constant availability of social media on smart devices has significantly decreased mental health and does significantly hinder the social and emotional development of children.
I know social media is bad for adults, and worse for children. That does not justify a ban. Parents can choose to not give devices to their kids, or lock them out of specific apps. Yes, the kids can circumvent that, just like they can circumvent this law with a VPN. So it's useless.
But it's worse than useless, it's harmful to adults and to the rest of the planet, because the only way to enforce this is KYC. And the cat and mouse game that follows will make that KYC ever stricter. We've seen this movie before.
Maybe a ban on a national level is a bit drastic, and I certainly do see the points you're raising about enforceability. However, I hope this starts an even wider discussion so the general message settles in and teachers, parents and other caretakers get the help they need to fight this epidemic.
Unfortunately I think the net effect of this overreach is that any such discussion will be overwhelmed by noise.
The Netherlands banned phones in classroom. Opsec concerns aside, that seems more reasonable, doesn't bother adults or anyone outside the country.
A school-based ban does indeed seem like the best option, even though it doesn't fully address the problem.
Putting this on parents alone might not be the right approach, individual parents can't fight a billion dollar industry that specifically designs their applications with armies of specialists to get children addicted to devices and applications. (Which of course doesn't relieve the parents of their responsibilities either.)
It's probably a way to push the digital ID rollout: https://my.gov.au/en/about/help/digital-id
