Same with me.. But usually I can racionalise things, amd realise that I would gladly change my situation with any of my mates who can speak fluently during presentations. But anyway, to avoid guilt, I usually ask the teacher or the professor for other tasks what I can do. For example, during the presentation sessions, I work with an other person who will speak, and I will collect the informations, make the slides, etc. Or just simply write an essay from the given theme. Don't be ashamed of things you can't do, even if it seems just an excuse for the others. Stuttering makes life much more difficult, don't do it even worse for yourself.