The grading policy at the middle school here is that grades are based on the actual learning, not behavior. The more I hear about your school the more toxic it sounds. Did you decide if he will be going back there?

If a disability affects educational performance (which it does, if they are grading based on behavioral factors), then the student should qualify for accommodations/modifications. Do they think the behavior issues are unrelated to the disability, or they would just be like this with any disabled student? Honestly, though, I don't think you are ever going to get through to them. If it were just the teachers, then you could maybe get somewhere, but it's also the support staff and admin.