Jon may have kept the stone on him, but he wasn't about to make any wishes. Not even during gym, or in the middle of class when he had the mother of all erections. He kept control of himself, or as much as he could.
School was boring through - even more when he knew he could just make anything happen with a wish. He planned some out even and wrote a few in a notebook (he was careful not to touch the stone then, just in case).
But watching Karyn navigate her knew life with her large (beautiful) breasts, he knew he needed to be cautious. He didn't want to make things worse for him or for her. He had to plan and play it safe. Even with all the power in the world, things could not work out.
Still, by the end of the day, he was itching to make a wish.