Be firm about it. Full stop everything else that is going on. If you're driving, you pull over. Get in their face and let them know that it's out of bounds. I am assuming you're male, and in your teens. Teenage boys are dicks. I know my friends and I were. I disagree with /u/Halt_127 about them not really being your friends. Friends hoot on each other. But if it bothers you, you need to put a stop to it. Recognize that there is no way they can possibly imagine what it's like. Who, other than a stutter, really understands what it feels like? There's no way they'll ever understand. But they can understand that you're not tolerating any more jokes.