Biff Meadows was the captain of the school football team and (more importantly) Sarah's boyfriend. Jon hated him and so did most everyone other than the jocks He was Sarah's boyfriend.
Jon figured that if he could successfully pose as Sarah, he could mess with Biff.
Jon closed his eyes with concentration, thinking to himself "I'm Sarah McMillan, I'm Sarah McMillan," over and over again. A moment later he opened them and checked his reflection. He still looked like himself in the mirror, of course. If the effect really worked, it would only affect other people.
Jon reasoned if he just headed over to Sarah's or Biff's house, he'd know whether or not the wish had worked soon enough. He put on his jacket and headed out into the evening air.
