The Japanese example is a very interesting one, thanks for sharing it.
Regarding parents, I can tell you as a teacher that many kids understand their phones way better than the parents do. Which isn't that surprising considering how much time they spend on the devices.
Some parent understand locking their kids out of problematic apps or features, or limiting their screen time via OS controls, but I wouldn't say it's the majority of parents yet.
But my point was that a default lock on everything potentially harmful is something that is both easy to legislate and easy for manufacturers to build into phones so parents only have to concern themselves with removing locks. I think that's a more practical and effective way to accomplish the goal in question, without putting people's privacy at risk, or counting on busy parents to figure it out themselves.
There will ALWAYS be workarounds for those kinds of things.
Since you're a teacher, what do you think about inviting parents in and then showing them how to enable parental controls on devices? Tell them kids are using phones in class and are distracting the class. Then you can mention that, rather than taking their phone away from them, which some parents would be against, you can show them how they can enable parental controls which would stop kids using social media or even the browser and even the app store. That way they can focus in the class.
Asking the government to step in will cause a HELL of a lot more issues regarding privacy than securing privacy.
At the end of the day, it's up to the parents to parent their kids and a teacher educating them about parental controls might open their eyes up.