What happens if I don't sit this exam?

I have the first part of my A-Level Sociology exam coming up in 2 weeks (media, and then crime and deviance a week after that). I have missed so much lesson time over the past school year due to illness (I have Crohn's disease) I don't know how on earth I'm going to sit it! I'm stressing out about it so much, I'm making myself ill again... I've spoke to the head of my college and he insists that I just go in and sit the exam anyway, despite missing so much and having done NO revision what-so-ever yet due to illness. But he said, if I do that badly I can come back for a 3rd year instead of having a gap year (I haven't applied for Uni this year because of illness, and am looking to apply for drama school), which I wouldn't mind! So my dilemma is, which I need help with... Do I just cram in as much revision as I possibly can in 2 weeks, and just sit the exam and come out with a rubbish grade (predicted an A)? OR do I just not sit it and stay at home? What do I tell the school then 'cause they're expecting me to sit it. Do I just say I'm ill and not do it, or what?
Thanks for any help you can offer