I believe that most school systems do get more money for disabled kids. But kids that do their homework, pay attention in class, etc. cost a lot less to educate than a kid who isn't really interested in learning--even without any disabilities.

To your point about charter schools, there is a strong argument to be made that when parents have a choice where there kids go, public schools will have to make sure they are providing more academic opportunities if they don't want to risk losing the academically minded families (and their money). That seems like it would be a good thing for everyone.