Haven't read everyone's replies, but the phrase that pays here with your school is going to be "duty of care."
JMHO. That is a clear message that you know that THEY know that they are legally liable for anything and everything that happens when he's theirs for the day. He was unsupervised for an extended period of time, and this was apparently completely unnoticed by any adult.
Bad, bad juu-juu.
If you feel that this situation is even potentially the creation of a hostile learning environment because of your (or your son's) advocacy efforts, that's CLEARLY against the law, and it's spelled out in Sec. 504. Retaliation might be hard to prove, but it's not hard to mention that you're thinking it's plausible as an explanation. KWIM?
(You mentioned ADHD-- I'm assuming that you have a 504 plan.)
