As an educator I can simplify it pretty easily.
We educate all, most countries do not. We water down our educated by continually pooling those that want it with those that do not.
Solution: You screw around in school, you get kicked out and work. Makes parents more vested in their children. Removes the need for standardized testing. Fixes a load of problems. Expulsion just doesn't happen and it doesn't remove them from education, just that school.