Jon wondered just what the payback could be. Of course, any wish that Kelly could make, Jon couldn't even remember, unless she wished that she'd remember it. Kelly did say that one of the things the revenge was for was for Jon spreading a rumor that Kelly was a transvestite.
Maybe Jon shouldn't have done that. She knew how difficult it was to be one herself, especially since she was trying to pass for male. She had to cut her hair short like a boy's, hide her breasts, and talk in a deeper voice. At least the asbestos scare in the gymnasium kept gym class cancelled for like forever, so she didn't need to worry about that.
In fact, she would have wished she really was a boy if she hadn't messed up her first wish. Jon had first tried "I wish I was a girl in the best possible male disguise" and only later had realized that that wish meant she'd forever be a girl.
But what could Kelly have done as revenge?