I would say don't speak to the classmates specifically in regards to this student. Instead, focus on respecting ALL students. Something along the lines of I'm noticing you all have taken to laughing at each other. We're all beginners here and there's never a reason to make anyone feel uncomfortable in my classroom. Maintain that this is YOUR room and YOU are in charge. This, again, is not limited to stuttering. If anyone was being made fun of, the teacher is expected to step up and maintain decorum.