"Johnny" is a codeword used whenever you're doing something you're not supposed to be, and there are teachers/police around that your colleagues need to be informed of. Rather than shouting "Teacher!" or "the cops!", shout "Johnny", and the afore mentioned authorities will not be drawn to the scene, giving your friends enough time to resume lawfulness.
"We're gonna beat this poor kid up. Who's on Johnny watch?"
"I'll look out for Johnnies for ya."
"No, you're the kid we're gonna beat up."