Your 13 and using better grammar then most adults. I've had auditoriums laugh at me, just keep talking eventually they will shut up and realize what your saying is more important then how you say it. Raise your hand in class when you want to say something, do the thing that you are afraid of. You may never overcome it but the important thing is to keep trying. I know it's tough having one of the few disabilities it's ok for people to laugh.You are going to have to develop a really thick skin and just know the people who make fun of you won't be your friend, but would you choose a friend that treats people like that? Just keep at don't stop talking. I've been stuttering for 30 years it can get better but first you have to not give a shit what people think about your speech.