specifically things that seemed to blow up massively at your school for what seemed like a short period of time

primary school -
wrestling/panini football stickers
rumours of child-killing clowns in a white transit van roaming the local area, even got a letter home from the head about it

secondary school -
pretending to punch someone in the arm then getting two free hits at them if they flinched, affectionately known as "two for flinching"
people going round with the lids from glass bottles of oasis flicking them in people's ears causing their eardrum to pop

now you go...