i think first thing's first is to speak to your parent(s)/guardian(s) about moving up into the mainstream classes. ask them if they can organize a meeting with you, them and all those relevant (special ed teacher, head of year, class tutor, poss. headmaster). if the school haven't got a good reason to keep you in the sp. ed class (you being diagnosed with an ASD does not count as a good reason), and you have high enough marks to back up the fact that you can work in the mainstream classes, it may be possible to try them out for a while, and then move into them if you're comfortable. or at least spend less time in them (once a week vs three times a week)
at your age it's totally natural to feel embarassed about being in sp. ed, about being different from your peers. but if you later find that mainstream classes don't quite cut it, don't be too ashamed to move back down into it. there is nothing worse than being in mainstream classes and struggling - i know that from experience. 
(with the incorrect diagnosis, has it been removed from your medical record, or have you seen someone to be reassessed? that might help in future situations where your "diagnosis" may be relevant.)
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