I got my mustache at age 13. By the time I got to be a senior, I looked pretty much like an adult. (I was never carded buying alcohol from age 17 onward. Imagine how popular I was. :)) At any rate, we had fairly frequent substitutes in our Journalism class, as the teacher was kind of a stoner.
After a couple of months, we figured out the teacher was bringing in friends and friends-of-friends as unofficial substitutes, because he didn’t want to bring his frequent absences to the administration’s attention. We determined which ones were actual friends of his, and which were FOAFs, and when we wanted to go home early (it was the last class of the day), I’d come in and tell the sub I was the teacher, I’m there after all, go ahead and take off. I only did it a couple of times, but it never created any problems.
It’s not like we had regular in-class work, anyway, as the point of the class was to produce the school paper, so an occasional early exit was enjoyed by all.
A better prank was pulled by the seniors when I was a junior. The weekend before the last week of school, they took a bag of ready-mix concrete and cemented shut the gates to the parking lots. The sight of the road jammed a half-mile in both directions with cars and buses waiting to get into the lots was totally priceless. We ended up with Monday off while they jackhammered the gates open. Truly, a moment of glory.
Other pranks, not on authority figures but very funny nonetheless:
While on yearbook staff, as copy editor, I made sure the names of several people I disliked were misspelled throughout the book.
Once, during a basketball game, I was sitting up with a couple of pals with a friend who was in the pep band, which occupied the center of the stands. The friend played the xylophone (not strictly accurate; it’s the similar instrument, with each metal note bar on its own little block, but I can’t remember the name). She got occupied in a conversation with a bandmate; while she was turned away, we carefully and quietly took most of the blocks away. She didn’t turn back until it was literally her cue to play in the next piece; she finished her conversation, lifted her rubber hammers to play, turned back, looked down while starting to swing the hammers, and froze in midstroke with a look of absolute confusion on her face, which melted into a perfect “Uh-- oh, goddammit, you guys” expression. Hee hee…