Except that this is _exactly_ what happens at public colleges as well. In fact, my public high school saw that I was fast at math and got me into college classes early, so it even occurs there to some degree.
Most colleges are competing for students; there are market forces. Public schools have a much stronger monopoly due to the difficulty changing location. Additionally public schools still cost money, if it was useless people would not use it.
By "privatized", I meant a system comparable to the one most education privatization activists push - public money follows the student to the school of their choice, which may be public or private.
