It was still a bit early, so not that many people were at the school yet. But that's what Jon wanted. He wanted to get there early in case the results of his wish became apparent. Then maybe he'd get a heads-up before class started.
Thinking that maybe it was a good idea, he had decided to leave the stone at home, in his bedroom. He didn't want to make any more hasty or just plain stupid wishes. But when he saw the effect of Jay's science project, he realized that maybe leaving his stone at home wasn't the best idea afterall. He wasn't sure if he could just wish Jay's project away, since that might be considered "reversing the wish", but he'd rather have his options open.
Jon was at his locker, opening it to put his school bag inside, when suddenly, he heard a loud high-pitched yell. It was the scream of a girl. He threw his school bag in his locker, shut the door, and ran around the corner towards the scream to see what happened.