I just remembered one from years back, but it turned out to be quite a big deal in the end, even though we didn't think much of it at the time.
We used to skulk off to the broom cupboard to skive our German class at times. One day, near the end of term my good mate decided it would be fun to record over one of the disused German language cassette tapes with a lot or lurid sexual innuendoes and language. He even went as far as inviting folks (listeners of this tape) to his house [address included] for some unadulterated pleasure.
A few weeks later his parents were called to school.
Unfortunately, it transpired that this "blank" German language tape happened to have the verbal GCSE language questions recorded onto it originally and was subsequently played to a class.
Okay, not so much an intentional prank, but funny never the less. I hope he isn't reading this. He'll likely get a flush!