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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A dude in my class in the 10th grade slipped in cum on the bathroom floor. No one used the potties again.