As others have mentioned, there are many factors entirely outside the school environment that effect a student's receptiveness when they are in school. Is their home life stable? Do their parents make sure they get enough sleep? Do they have a quiet place to study? What messages do they get from their parents about the importance of education (where I live many are intimidated by their children getting more education than they had)? I would love to see parents held more accountable for their children's success in school.
Genetics, of course, enters into it. If teachers are paid based on the success of their students (similar to a sales commission), no one is going to want to teach special ed, where progress is often a struggle, and everyone is going to want to teach bright students with ideal home lives. It's not good for students (reinforces looking with ill-favor on slower students who could impact one's financial status) and why should a teacher be financially punished for working with more problematic, special ed students?