Back when I was trying to get out of a day of grade school, I devised a pretty solid plan.
There was a rusty wagon under my back porch. So what I did was I snuck out early in the morning with a cup and took some of the rusty water inside the wagon inside my house. I poured it in the toilet and told my parents that I threw up. I stayed home that day for some reason.

My mom told me a story once where one of her friends faked falling down the stairs to skip school. The plan backfired and he really DID fall down the stairs. His mom felt so sorry for him-even though she knew he meant to fake it-and let him stay home for the day.