It would be funny if it weren't true
It's not so difficult...
I won't be able to teach tomorrow because I'm on sick leave. The school has already asked for a substitute teacher, but I don't know when he or she will be there. Pay attention to your mobile phones because you'll get news about all this in the next days.
here's a suggestion of something you can do while there's no substitute teacher (and continue doing when s/he replaces me): you can read an original novel, which can be challenging but quite rewarding once you've done it!
Why gunpowder treason