Does the friend know you stutter? The teacher probably didn't know you had a speech impediment and thought you were being nervous and didn't know what to say. I'm sure if people knew then they wouldn't have laughed. If I was in your situation, I'd let them know that I had a speech impediment and that I'd had no control. For those who continued to smile or laugh, I'd be self-assertive and ask them (bringing them to the spotlight) "What's so funny? You're laughing at something I have little control over. Would you laugh at someone who had cancer?"