Honestly, judging by your posting style if you were to actually be autistic, it would just be in the social aspect of things and a very mild case at that, from what I can gather.. Basically, a bull shit diagnosis.. More than likely you were just awkward for a time, your mother didn't know how to deal with it, and she sent you to a shrink instead of actually parenting you.
My suggestion would be to just talk to the teacher after class one day. When they notice that you don't talk retard, chances are they'll see your mother's letter was mostly hype.
On the bright side, you parent seems like she may be ineffective, but I also think that she actually cares. That is always a plus.