Want to skip on school? Just be sick for real - easiest how to do it? Eat and apple and drink Cola at the same time - guarantied stomach ache of extreme magnitude inside the next 12 hours. Not sure if quantity has anything to do with the result. The actual experience involved 1 apple and I think 2 dl of Cola.
Source: Me. Yeah I was enough stupid to do this - trough I didn't know that it would result in a stomach ache. To bad it was during a school break so it just ruined my day instead of giving me a day without school.
Of course this is only if you are really desperate and know that your parents/boss will not buy you faking it.

But again you probably shouldn't skip at all. Skipping is bad. Really bad ...