I'm 14 and was just recently diagnosed with tourette syndrome. Only my close friends know about it, and they can sometimes be a bit ignorant of the condition, too. And they laugh at the fact that I'm on medication for it. It's so annoying, because when they laugh when I talk about it, or they see me taking it, or whatever, I would say, 'Would you laugh at someone taking medication/treatment for diabetes?' And they would say, 'Well diabetes is a little different', while laughing. But it really isn't. I was born with this condition, just like people with diabetes are. And I'm treated for it, and so are people with diabetes.
What you could do is bring them to a tourette syndrome conference (I tihnk one is coming up soon), maybe show them the HBO documentary 'I Have Tourette's, But Tourette's Doesn't Have Me', or the movie 'Front of the Class'. Or maybe even ask your psychiatrist/neurologist if they would be willing to explain the condition to your friends.
If they still don't believe it, or won't even try to believe it, then they aren't very good friends. I would tell them this, if your comfortable with it, but only if all else fails.
Good luck and I hope I helped! :)
Fellow touretter. :)