Humiliated!!! :(

LegacyUser

Theres always one in a class, someone who loves the sound of their own voice, constantly annoying the teacher , as for answering the phone and not leaving, lol, that is ignorance at its best.
I can assure you most people were cringing or annoyed at HER when she did that, ive been in classes like that , and the best part about your situation is, if she really is that rude in general people have already formed their opinion meaning even if she was right this one time people still probably got annoyed at her!! (even if they did acknowledge in their own mind that the phrase probably wasnt the best. )

You learned a big lesson about your writing though. Because while mildly retarted might be a technically correct term it comes across as ignorant, unresearched and slightly lazy writing, even if that wasnt the case. It seems to me that if you were describing someone with a slow learning ability you should probably have elaborated more on it(maybe you did I dont know), as opposed to just saying 'mildly retarted'. Itd be the equivalent of describing a person with some sort of physical condition where he couldnt walk properly as a 'cripple', i.e. its technically correct but lazy writing and slightly offensive.
But I mean to write that in a positive way, i.e. you LEARNED something regarding your writing technique and now you can improve on that.
Personally I wouldnt bother saying it to the teacher, its pointless and just putting them in a slightly awkward spot, they're not going to join in and start going 'ah dont worry shes just a rude bitch' even if they're thinking it.

As for going to this class to help overcome shyness, well done, that is exactly what you should be doing, keep it up and dont stop there, start getting used to the sound of your own voice out loud. Go to more classes!!
I can assure you if you started a post here saying 'im shy, what could help overcome this' there'd be a few people suggesting for you to attend night classes in various hobbies like that.