In 5th grade inside one of the boy's bathrooms, there was a giant turd in the middle of the floor nowhere near the toilets. It smelled pretty bad in there too, probably because of that turd. There was also poop smeared all over one of the stalls.
Other than that, there hasn't been anything really disgusting ever since.
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In my primary, there were two little kids peeing in the communal trough once. One turned to the other and peed on him and laughed. The other laughed and peed on the first one. They then both ran out laughing. At the start of high school there was a kid who stole someone's hat, took a crap in it and then gave it back. He also laid a crap on the concrete near a set of stairs for no apparent reason.