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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Well, this one time I had to take a really bad shit at school. So I went down to the nicest bathroom, and I had this massive shit that I invented a new word for. Combine shart and diarrhea. Shartarrhea. I couldn't control it so it basically splattered all over the toilet and wall. Amazingly, none of it ever got on me or my clothes. I'm sure many people were disgusted by what I left in that stall.