Elementary School:
I let out a massive fart in class (purely accidental, I tell you), and convinced everyone I made the sound with my mouth. I got in trouble with the teacher for having "no excuse" for "intentionally disrupting the class", but at least most of my classmates believed that it wasn't a real one, saving my reputation.
Middle School:
I bet most of us at some point gave some lame excuse for skipping Mile Day in P.E. class and managed to get off the hook, though we always had to make it up later.
High School (senior year):
"I'm not ditching school. I attend the community college across town. I'm just here to see one of my old teachers. Excuse me." I hurried back to the class I was supposed to be in. Didn't get to escape my stupid sixth period class (a useless P.E. class mandated by the school), but at least I didn't get in trouble.
College and University
None. For some reason, I never had a reason to lie about anything. It was just a matter of going to class and going home. Then studying, then returning to class. Serious business.