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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At my current school, since there is a lot of wildlife around here, animals like to come in and shit in the courtyard and stairs. Note, the stairs are outside, not inside.