I would just say "I don't feel well". :P I suppose you could claim you threw up, and flush the toilet, and no one would know...

I don't plan on missing any school this year though, and last year I was only sick (and not really very sick at all) for one day, in the second week of the year.

Really, it's usually not worth it, especially now that I miss so much school for extra-curricular stuff.