Later they only get ability to sit at the same classes at the same public school, so there is no financial incentive.
15 year olds forced to sit in classes they don't want are way more miserable than those allowed to work and feel like adults. In any case people should be allowed to make choices by themselves not be forced by the government.
> the school I went to had special classes for unruly kids
That's a great solution too, and must be available option for parents. Sadly very few schools do that, making both unruly kids and good kids miserable as a result.
> schools should be given the resources
I don't think the problem is the lack of resources, specialist for helping unruly kids is not going to cost more than a math teacher. The problem is that most schools are simply opposed to the idea of splitting students based on their ability and willingness to study. As a result they have a system that harms everyone involved.