Professor Poopypants: Hiyah class, I'm your cool new teacher, not some scary guy with a secret evil agenda.
Harold Hutchins: Is it okay that I'm kinda loving this?
George Beard: Yes and no. But mostly yes.
Captain Underpants: Now I take to the sky like an ostrich.
Professor Poopypants: Hello! I was just, like, admiring the view from your... broken window, in the shape of a naked balding man.
Melvin: Oh, Professor Poopypants, George and Harold are trying to stop you.
George Beard: When I snap my fingers, you will obey our every command. You are now the greatest superhero of all time, the amazing Captain Underpants!
Mr. Krupp: Tra-La-Laaaaaaaaaaaaaaaaaa!
Answer: They never would have found him after CU slapped him far away. Plus Harold destroyed the shrink/grow machine so PP ended up permanently small, meaning someone could easily squish him if he tried anything evil again.