Jon picked up the stone, and made her wish. "I wish that my last two wishes happened the way I meant."
Immediately, Jon was a boy again. Whew, he thought. The stone was working fine after all. He'd just have to be careful how he worded his wishes.
"I wish I knew why I don't have school today." There was no way that the stone could misinterpret that, was there? The wish came true, and Jon realized he knew... but then it completely slipped his mind. He couldn't recall what he was thinking a second ago.
Needless to say, he knew why he doesn't have school, in the past, but never retained the knowledge.
"Argh! I wish that worked the way I want too!"
Suddenly, Jon felt a great desire to be a girl. He wanted to be one more than anything in the world, and the stone, giving him what he wants, now transformed into a girl.
Holding onto her flopping breasts, Jon said "I wish that happened the way I want to!" Jon was ready to throw the stone away if this wish got twisted too... but it didn't. Suddenly he was a boy again, and he knew that the reason he didn't have school was a gas leak, that should be fixed by the next day.
Every time he made wishes, in fact, the wishes would keep going wrong... until one of them went wrong in a way that turned him into a girl. Since he was then a girl, all his wishes would work, at least until he picked a wish that changed him back and started it all over again.... Would Jon ever figure it out?