Why not lock device/accounts as minor and put onus on school and parents to ensure devices are appropriately tagged? At least for pre-teens I strongly think it shall work.
Because it will take about 1 month till there is some service the parents will want the kids to use that wont be available on such device (a kids show, a kids game, a page necessary for homework). So, they will have strong motivation to not label them as such.
At that point, what if parents just let their kids borrow their driver's licenses to use social media? There's no technical solution to bad parenting.
The only reasonable solution that doesn't infringe on privacy is to give parents the tools to limit their children's internet use, and presume, outside those bounds, that people are adults.