There was one thing neither Kylie nor any other student at the school knew about Laura, because she was very careful to hide it. She was a geek.
Specifically, a comic book geek.
Ever since she was a little girl, Laura had loved superheroines. As a high schooler, she realized that loving comics was about as far from being "cool" as it could possibly be. She drove to a comic book store fifty miles away every Wednesday, to pick up the new comics in a place where no one from her school could see her.
So when Kylie had trapped the partygoers and ranted about what she would do to them, Laura saw her not as a sorceress, but as a supervillain. The story logic that had been drilled into her since childhood told her that a superhero or superheroine--Laura hoped the latter--would arrive to rescue the supervillain's victims. She hoped she would play a part in the story, a thought that was responsible for the sexual arousal Kylie had noticed.
Kylie was technically right in saying he wouldn't change Laura. She would change herself, as he had given her a strictly limited power to transform, restricted only to herself.
"She becomes the real Wonder Gal, only immune to magic" said Laura.
Suddenly the twist in Laura's perception disappeared. She looked at herself amazed, as she felt the superpowers of the heroine coursing through her body. Both her muscles and her breasts had grown to those of a female comic book powerhouse.
"I'm a superheroine!" exulted Laura. And as such, she fought supervillains, like the sorceress who was looking like she had somehow lost control of the situation.
With superhuman speed and strength Wonder Gal--Laura--threw a hammerlock on Kylie. "Release Jennifer and her friend." she whispered in Kylie's ear.
"What if I don't?" answered the sorceress. "I can turn everyone here into a monster and have them attack you."
"Before I break your back?" answered Wonder Gal.
Kylie turned pale. "You can't do that. Superheroes don't do that."
"I'm the grim and gritty Frank Miller version." answered Wonder Gal, giving the arm twisted behind Kylie's back a tug. "Now do it."
Jennifer and her boyfriend looked away from the mirrors, and the delight on their faces was replaced by profound shame.