The school will call your family to pay their regards because you're either dead or close to it.
Just joking, but you have to pretty much dying to not go to an exam. Even if you are very ill, you should go to the exam, notify your year coordinator beforehand about what's going on, do your best in the exam and basically go right from the exam to the doctors- this is VERY important, because you can't lodge a misadventure if you don't have documentation.