By the time Jon had completely stood up Biff had made his wish it was clear, Jon had become much thinner and lost a lot of muscle mass.
Biff wish was "I wish I was the strongest guy in school, in fact I wish I was twice as strong as any other guy in school.
Instead of Making Biff freakishly huge it made the other guys pathetically weak, probably weaker than most of the girls, Karyn looked like she had more muscle than Jon did right now.
They couldn't tell if Sarah liked his wish or not, all they new was that a few moments later Biff made another wish. I wish I was the only guy at our school and the girls all had porn star bodies.
and before they could even blink Sarah made a wish, I wish you were back home and forgot about this stone, they guessed Sarah had quickly noticed that he shouldn't have something that powerful and swiped it.