before french class at around 15 y/o we all set our alarms on our watches, 'personal organisers', and the classroom clock to the same time in the middle of class. We put them all around the room, behind lockers, in teachers desk, outside windows, hanging from the cross above the white board...etc
One was set for 10mins later which i had in my bag beside me.
One by one they went off, my french teacher went crazy! he called the headmaster who came up and called everyone involved outside the room, at that exact time mine went off..i got detention didnt turn up and went to watch football instead!