Trust me, I know how it feels like. People can say to you (which is the best peice of advice) "don't worry, it's not worth worrying and you'll be fine" but you can never feel that way about it. But think, the last times, did anything bad happen? It may be hard to beleive, but sometimes things go better than we think they will do. Secondly, pracice it in front of the mirror, make sure you know it well. Try to look happy, and IF you make a mistake, smile, laugh and just go with. Don't get red and shrink away to nothng. But remember a very good thnig my brother told me:
"Bad things happen when you expect them the least"
I'm always worrying, about lost school books, homework and those things. I get extremely worried but it all turns out okay. Even if it does go wrong, it still dosen't matter much anymore.
Lastly, don't always look on the negative side of things. If you do this like I used to, every bit of forgotten homework led to being sent to the headmaster and put into the inclusion unit for a week! Always think positive. You may doubt it, but trust me, it works.
But if you absoloutly CANNOT do it, talk to your teacher. If not in front of the class, ask if you can him at the end of the lesson, or when he has time privatley. Tell him you get really really nervous. it may be hard, but the only other way is lying; and we all know lying is immoral. He will have the best advice as he has probably dealt with loads of other people like you who get nervous alot. I know last year I asked my teacher if I didn't have to do it. He told me excatly what I have said above and now, I don't mind at all. In fact, I enjoy them. Stay happy, strong and courageos and you'll be fine, I promise!
Good luck! :)