1. Government overriding parental rights in a state that is so proud of all possible "my rights"? I suspect some challenge in court for this law will happen sooner or later.
2. Closed circuit networks with some "private" server to chat between friends might get popular soon. There are enough quite capable 13y/o kids able to do so. Or 14yo, creating one for their little brother.
3. Not having a personal account might not prevent anything, as using parents' account is still possible.
4. Whole class on the same Signal chat room -- is it a "social media"?
5. Somehow parallels of Twitter getting banned during Arab Spring events come to mind.