I did Model UN in high school as someone who stutters. My main advice is to do a lot of prep work so you feel confident in what you're saying. My larger advice is to look into Avoidance Reduction Therapy. My life has improved drastically after I stopped caring about stuttering and stopped being embarrassed about it. Ironically, I rarely stutter now but when I do I just move on. ​ I've found that if I stutter a little bit at the start and get the embarrassment over with, speaking more fluently gets a lot easier. ​ If you don't want to participate though, then don't.