Somehow it is impossible for people to blame the system, but instead they diagnose physical deficits in children based on their inability to adjust to the system.
Maybe the random way we chose to mass educate children a couple hundred years ago isn't perfect, and children are not broken?
TL;DR: Disability is not inherent in difference, but rather a combination of the difference and an environment in which that difference is not well-supported. For an analogy - a deep-sea fish is blind, but not disabled by their blindness. Similarly, a kid who "can't sit still" isn't disabled unless we put them in an environment where they have to.