I had friends from highschool that laughed at me when I stuttered. I'm still good friends with them now. And they became patient with me and never mocked/laughed at my stutter again. I never confronted them or voiced out my dismay. They just stopped making fun of me. We're 30 now. And still hangout almost every week. Your friends are just immature. If you think they are true friends, brush it off or treat it lightly. Them laughing at you is just them being young and immature.