It was Wendy, fortunately, Jon explained his situation, and she thought about it... and gave him a plan. It might not work, but... just standing around certainly would not work.
Jon hid in a space at the end of a row of lockers, waiting for Sarah. Soon, Sarah came by, and Jon jumped on top of her. He couldn't jump all that well, but his weight made it hard for Sarah to get him off of her.
Sarah, who was quite dextrous from long hours of cheerleader moves, finally managed to slip out from under Jon. "I see you still want my stone."
"It isn't your stone," he replied.
"It is!" Jon tried to jump on Sarah, but instead hit himself against the side of a locker. Sarah took the chance and poked Jon in the belly. He gained ten pounds and lost an inch of height.
"Go ahead!" yelled Jon. "Keep poking me!"
Sarah poked Jon several more times, and then said "Listen, you useless lump of flesh, I know better than that. I wish that when Jon is less than an inch high, poking him will no longer have an effect." She laughed wildly and poked Jon dozens of times.
Of course, Sarah wasn't as smart as she thought. The wish did nothing--she had wished that poking him in the belly would have that effect. Wishing that it would no longer do so under some conditions was reversing a wish. The stone didn't allow that. So Jon continued to get shorter and heavier, until he was an inch and a half tall and hundreds of pounds of fat--and two more pokes from Sarah compressed Jon to twenty pounds more than that and zero height.
The resulting rather flat quantum black hole exploded in a burst of radiation, killing Sarah instantly. Fortunately, for complicated reasons that only a science geek would understand, it did not also destroy the city.
Fifteen minutes later, Wendy returned to school long enough to ask for readmission--or so was her excuse--but what she really did was take the stone off of Sarah's body. "I wish Jon was alive, awake, and not producing gravity or radiation."
What was now left of Jon was an infinitely thin, conscious, several hundred pound sheet of flesh on the ground. Jon thought to himself "I'm going to tell people about Sarah."
The resulting transformation was slow, painful, but a bit pleasurable. Sarah had made dozens of wishes since her first wishes on Jon. But even dozens of wishes could not reverse the first few ones--including the wish that whenever Jon thinks of telling people about Sarah's powers, he'll turn into his own image of a beautiful woman.
Wendy reversed the wish Sarah made on her (since its exact wording was never revealed, neither will this story reveal the exact wording of the reversal), made herself rich on top of it, then gave the stone back to the beautiful woman that Jon had turned into.
"Thanks, Wendy... though it looks like I've got a lot of work to do," said Jon. "I think I can just wish away the effects when Sarah used magic powers, but she also had some real wishes in there... What can I do about Ethel?"
