J.B.
S. -
I am so sorry you are having difficulties at your school.
I would imagine that you have an IEP (Individualized Education Plan) in place for your kiddo - of which includes that he should be taught in a least restrictive environment. Your assistant principal's attitude is not being 'least restrictive'. When children are placed in environments where they are automatically expected to fail (as with your assis. principal's), they often perform worse than when they are placed in environments where they are expected to succeed. The next time your assis. principal decides to make a comment like this, I would remind her of this fact.
You can give her pamphlets/handouts of your son's disorders to educate her on the facts as it seems she does not understand that it is 'real' but something he is using as a tool to manipulate you/the school system. Invite her to the next IEP meeting you have so that she can hear positive information from the teacher.
And if all else fails, go above her and talk to the principal. Her attitude is not constructive to the situation and could be making things worse.
As for your little one - if he thinks he is dumb, he can start falling into a 'self-fulfilling prophecy' in a way... not trying as hard because he doesn't think he'll succeed. Try to get him involved in activities (school-related or non-school-related) where he can see himself succeed. Something as simple as a playdate with a friend he gels with, swimming, kickball, rollerskating... whatever it is, he needs to feel he has self-worth. And remind him that intelligence doesn't always mean book-smart. There are a lot of people in the world with tons of 'book knowledge', but that doesn't make them 'smart'.
Good luck with your situation at school and with your little guy. Mine's only 3 months old and I'm already not looking forward to the pre-teen years. :)
J.