I know this is hard at your age but lots of your peers will also have nervous stomachs that day, so try to laugh it off if you get the grumblies. I am fully aware of how embarrassing it feels to have fellow students know entirely too much about your GI tract (I had to leave an anatomy lecture twice today to go have diarrhea; it's hard not to be conspicuous when you're leaving the middle of a huge lecture hall) - but the more you think about it, the worse it will be. So definitely don't worry too much. Besides, those other kids will have much more to worry about than the sound of your tummy!
Beyond that, eat a light breakfast, go easy on the gas-producing foods (broccoli, etc.) for a few days before the big day, take a Beano or something to relieve any gassiness, and kick some butt on that test!