This happened to my friend G, back in the 1960s. He was 14-15 years old, in high school, in a small west coast town, USA.G was a small, easygoing kid. He did some work - cutting lawns or something - for his neighbor Rich Douche, same age.
And of course RD stiffed him. G's parents complained to RD's parents, who couldn't believe their little perfect son would ever do such a thing. 

 There were a couple of other incidents - mostly RD stealing credit at school for work G had done, or general bullying. But RD was a suckup as well as a weasel, and nothing ever happened - he wasn't even suspected.Key point: RD had an electric typewriter - rare in high school - and bragged about it and showed off his efforts; he claimed to get better grades on typed papers.

So, revenge. 

 Like most of the houses in the town in that bygone era, security was minimal. It was easy to slip in an unlocked window, so that's what G did when the house was empty. He brought a little file with him, and filed a little off the bar of the "e" character on the typewriter - just enough to make it unique. Then he (slowly) typed up a page - G didn't type so it was hunt and peck. The page started with "Ha ha the stupid teacher never reads this far. He's a real dickface..." and continued for a full page; it used every dirty word he could think of, and especially insulted the vice president, in charge of the teachers. He then left with this piece of paper, leaving everything else untouched.Several weeks later, RD had typed another paper; it was paper-clipped together per requirements. G somehow managed to distract either him or the teacher, and slid his special sheet in the middle.RD failed the class, though he'd been getting good grades up to that point. He appealed to the vice president; the extra insults directed VP's way made the VP allow the teacher's grade.RD claimed that someone else must have typed that - but upon closer examination, the modified "e" character matched on all pages, and didn't match any other typewriter. Checkmate.His own parents didn't believe him. Once they started looking, they found a lot more bad behavior that they'd ignored, and RD ended up in military school - NOT the place you wanted to be in the 1960's.