Faking sports injury? Genuine reason!?

I've got my GCSE exams coming up soon, but my cricket coach says that we've all still got to play in matches, on Saturdays, even though we've got exams. Our headmaster's backed our coach up as well, so there's no point just refusing to play. Seeing as I've got 22 exams coming up over the next few months, I really need that time on Saturdays to revise. So, does anybody have any ideas on an easy injury to fake, which would last about 6-8 weeks, until my exams are over? Thanks in advance.