I don't get it either, but people have laughed about my stutter in a non-malevolent way too. They didn't keep in mind that it's not deliberate, I guess also because my stutter is often not visible anymore. I got mad too and told them why it's not appropriate. Especially in a professional setting it's not appropriate. I'm sorry it happened to you :/ I think it's great how you stood up to him. I would also stick with the class and not report him. It's not worth missing a class you want to hear. Unless maybe you have to talk lots in this class and this incident will make you stutter all the time. If he kept being weird about it I would report him.