I've never sent a useful email before in my life. Does this sound ok?:

Hello Sir,
I thought i ought to let you know I won't be coming in to school today, owing to a nasty knee injury i obtained yesterday. The doctor it's only a sprain but quite a bad one and i need to rest my leg for a few days. I'll see my teachers on monday to collect any work i missed today. If there are any problems please let me know and i'll try and sort them out from home as best i can.
Tilly