My class at Secondary School (the top stream) were collectively renowned for being practical Jokers, One of the pranks was to set off an alarm clock in the class when the new nervous French assistant was delivering his first lesson. We all leapt to our feet and panicked about, convincing him that it was the fire alarm, left the room two by two at quick march and assembled in the playground, while the poor Frenchman took the register to ensure that the whole class was out, but of course a few had hidden inside. Along came the janitor (who came from Aberdeen) who grilled him in Doric Scots as to why we were out, ensuring even more confusion.
We also sent Valentine Cards from one random teacher to another, drove our English Teacher mad with outbreaks of suggestive mis-spellings and many other ploys.
However our best, most successful plot. which we kept going for two weeks, was the Steradent Tooth powder one. My Mum and Dad both had false teeth so I supplied the powder. A small quantity was issued to most members of the class and just as the Teacher entered the room we sniffed the powder up our noses. This causes a, not unpleasant, fizzing sensation, which in turn makes the sniffer sneeze prodigiously, and even foam at the nose. Poor Latin teacher was at a complete loss as to what had caused this phenomenon and rushed around opening windows and issuing us with sheets of toilet paper, We repeated this in several classes, sometimes at the beginning, sometimes when the teacher had left the room for some reason and returned to mayhem. Many of us had friends or siblings in other classes who joined in so suspicion did not fall directly on our class. The entire building was given a thorough cleaning, teachers were banned from wearing perfume or aftershave etc. which must have worked because the mysterious sneezing outbreaks petered out. (actually we had run out of Steradent and got bored with the joke).