You go anyway and do the best you can. Nothing good can come from avoiding the slings and arrows, trials and tribulations of outrageous fortune.

I feel for you though; I had to give a talk to the entire school when I was in high school, one period at a time (all the history classes that day) and we were in pairs and my partner did the call-out sick trick so I was "along". We're not alone though, other people are counting on us, the teach, the other students, everyone needs your experience, even your fear and difficulties to add to their sense of what it is like. Do you think a real lawyer isn't scared the first case he tries in court? You can't get practice unless you practice. And practicing being sick just gives you practice being sick! What do you want for yourself? Experience communicating with others or being an A-1 responsibility avoider?