Thanks, Guest, blob, (!) I should have gotten back to my own request here. A couple of years ago, I had occasion to ask my original source about this and she told me how the verse went, which is what you have given us. From Toronto:

John, John the Leprechaun
Went to school with nothing on.
Teacher said, "That's not fair
Give him back his underwear!"

John, John, the Leprechaun
Went to school with nothing on.
Asked the teacher what to wear.
"Polka dotted underwear!"