I had to give a presentation last year when I was in 2nd year (14 yo) and I also did it on stammering. We had to talk for 3 minutes but I think I may have gone over a bit, but I talked to my teacher before hand and she said she’d allow me to go over if I needed to. Overall it went really well and I’m so glad I did it I still have the notes from that presentation at home if I ever need them again