Normally you phone the school on the day to let them know you won't be attending, and bring in a doctor's certificate, and if you get back at school within a reasonable time period, you re-sit the exam. Otherwise, they may choose to take an estimate...
I was off school for 2 weeks with whooping cough. The first day away we had a maths exam, because I had so much time off, I still re-sat the exam...but they won't use that mark. They're going to estimate it on how well I do in the trials...argh dammit >< not good.