Yes, you say, "you guys might not know this, but I have a speech impediment, it's called stuttering. I can't control it." Usually this puts people in their place. They feel embarrassed for laughing and often apologize. More effective with adults than kids, but you said the teacher laughed too. That's unacceptable, and unless he or she is a monster, he or she will realize they screwed up and do the right thing. It's their job to set the tone in a classroom; laughing at a student for their speech is not something that any teacher should tolerate and is likely against the school's policies.