I learned it as a kid in the early 90's from my parents. They were born in 1950 and would have been way out of school in the 70's.

When we sing it, we don't do any pfft or anything. it is:

When you're climbing up a ladder
and you feel something splatter
Diarrhea Diarrhea.

When you're taking off your dress
and you see a big brown mess
Diarrhea Diarrhea

Those two are for sure from my parents.