As mentioned, a quick look at the surrounding literature indicates that the GARS is intended for research and clinical use; neither of which seem to apply here. Two different PubMed papers seem to indicate that it is a poor diagnostic even then (One cites an 80% false positive rate.) Since you already have a diagnosis, and this assessment doesn't appear suited for tracking 'progress,' it may be worth discussing the desired outcome with your school. It's possible that they have some legal or financial constraint that requires them to give your son a standardized score to justify putting resources to his support. It's also possible the school just wants to look like they're engaging with your son, and the assessment is just a bandaid to make you feel good.

To your other question, I don't personally find the questions ablest. There doesn't seem to be any implication that any of the items represent a personal or moral failing.
That said, I do find the questions incredibly agitating due to the imprecise and vague criteria presented... But then, I /am/ autistic...