We need to normalize calling people out for shit like this. I get the "oh did you forget your name?" thing a lot. My response is "no, I did not forget my name. I stutter, but thank you for pointing it out." Most people will feel super guilty when called out like that (and honestly seeing this guilt is delicious). I would have a talk with your professor and be like "that was not okay that you not only allowed the class to laugh at me , but that you joined in." Good luck dude, don't let it get you down too much. Remember, we own our stutters. Our stutters don't own us.