You might look at Asperger's Syndrome. It's pretty hard to tease out the high-IQ end of the autism spectrum from that mix of ADHD-plus-sensory-with-anxiety-behaviors that you described. The anxiety from AS can push a kid to overload-type behavior, which can look like anger or frustration or inattention. Then again, the ADHD dx may be spot on.
A really complete diagnostic workup by an autism specialist or developmental pediatrician would include IQ and achievement testing but also might get to the root of the behavior you're seeing and could be useful. I wouldn't go to an OT for this (which is the standard route for "sensory" stuff).
Regardless of dx, if he has severe enough behavior problems to be escaping from the classroom frequently he probably needs the legal protection of an IEP instead of a 504. There are laws that protect a kid from being suspended more than 10 days a year if the behavior is due to a disability. In your shoes, I'd ask the school in writing to re-evaluate and ask for an IEP.
I hope next year brings you a better teacher.
DeeDee
