Things like that absolutely do not fly in the states. If you're comfortable, you can have a talk with the teacher about it. Even if you don't think it's a big enough deal to have a conversation, this teacher needs a wake up check that they need to be more aware and things like that are not ok. If you don't feel comfortable, I would tell your parents and maybe they can contact this teacher. If nothing else, this is a teaching moment for someone who is ignorant of stuttering and could use some education about what stuttering is, how it's not something you can control, and how jokes about it, especially from an authority figure can make someone feel.