Diarrhea. It will get you out of anything, including high school classes. No one will argue with you, or question you further.
(a teacher and a student)
T: I'm going to write you up for skipping my class again!
S (in a hushed voice, The Excuse): ummmm....I couldn't get off the toilet...I had diarrhea...
(at this point the teacher dosn't want to listen anymore, and pretends to be busy. you walk away in sucess)