Clutching the stone (which was actually, unknown to him, one of the Twin Stones), Jon said "I wish that she was bisexual." When he said "she", he meant Sarah McMillan, since that's who Jon and Greg were talking about right before the wish.
The stone glowed granting his wish.
At that same moment, the stone in Sarah's pocket also began to glow, as she turned bisexual. Unfortunately, that didn't make her want to join Greg and Jon anymore than before. Now, if Jon was a girl, perhaps she would.
Unknown to most people, Sarah McMillan was bisexual and was very attracted to the female form. More than anything, she loved girls' breasts, how they jiggled and bounced. It was mesmerizing. Unknown to Sarah, however, the only reason she thought that she was bisexual was because Jon just wished for it less than a minute ago. Since she didn't hear the wish, her memories had changed to fit the new reality.
Also at that moment, someone else in the high school was changed. Because these were the Twin Stones, the wish was granted to both Jon and Sarah. But if Sarah had wished that same wish, then the word "she" would probably have been in reference to someone she was looking at when the wish was made.
And right now, the "she" that Sarah was looking at was ...
