If its really that bad then I say don't do it at all. When I was in school we had things where people would perform on stage and I've seen it too many times where people break down on stage. <br />
But really, be realistic about how good your voice is.... Is it listenable? If so then make sure you believe in your voice. And to overcome the fright then practice over and over each time in front of more people.
Record your performance at home and play it back.