Jon knew he should stay. He should have burst in and demanded an explanation. He should have told Biff to get off of her (or out from under, but that lacked the ring). He should have done something. After all, if he was right he was going to have to do everything he just saw -- and more if he didn't put a stop to it. Instead, he ran to the boy's room and threw up.
How could she do this to me? I would never do that! (But I'll have to...) This is sick! I'd die before I'd let a man fuck me like that! ... That means my first time will be will Biff, as a girl. These were just some of the thoughts going through Jon's head as he leaned up against the stall wall, staring into space.
After a while -- he had no idea how long -- Jon came back to himself. He was letting things get out of hand. He didn't know that was really Jennifer. Heck, he didn't even know whoever it was was real. Not with the stone around. Somehow, Jon managed to channel the trauma of seeing himself moaning erotically while being taken on a table into anger, and that got him moving. He had to get the stone back.
After rinsing his mouth several times (stale vomit tastes awful) Jon retraced his squishy steps to the music room. The lights were still on, and Biff had gone. Jennifer? was sitting against the table in the back of the room, her head down and knees drawn against her chest. She was sobbing quietly.
Now Jon had no idea what was going on, but he could clearly see there was a crying naked girl in the room. There was something he could do about part of that, at least. He saw a jacket someone forgot lying on a chair. It looked just like one of his. He picked it up. There was a bulge in the right pocket...
...and in that pocket was the stone. This was his jacket! Had he left it in the music room? Never mind that right now. He draped the jacket over the girl, seeing far more sexual fluid than he was comfortable with in the process.
The girl looked up at him, tears streaming down her face. She did look exactly like Jennifer.
Then she said,
[ Apologies for messing up the italics and bold before. I give up ;) ]