Guy.
One of the kids who had been bullying him for going on four years now. And you were completely naked, with your nine inch penis hanging down. And as you start to realize that you've swapped bodies, it starts to stand to attention. Within seconds, it's erect. You blink in surprise and gaze down at it - pinching yourself to make sure you aren't dreaming. You aren't.
Reaching down, you touch your new penis, and slowly begin to rub up and down. You look back up at your reflection, and it still creeps you out that you're in Guy's body and masturbating - and it, it's what you've always wanted and it just feels so... right.
And when you come, it splatters all over the mirror. You got clean it up, you were always a clean freak... but that was you in your old body. You don't care anymore, you're Guy, Mr. Popular.
Oh, you are going to have so much fun...