Jawbreaker

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The "Flawless Four" are the most beautiful and popular girls in Reagan High School in Los Angeles. The clique consists of Courtney Shayne (Rose McGowan), Marcie Fox (Julie Benz), Julie Freeman (Rebecca Gayheart), and Elizabeth Purr (Charlotte Ayanna), the "Princess Di of Reagan High." Of the four, only Liz is genuinely kind-hearted to everybody regardless of their social rankings and loved by the entire school; Julie is popular because of her beauty and being best friends with Liz, while cold-blooded queen bee Courtney and her airheaded right-hand girl, Marcie, demand respect through terror.
Courtney, Marcie, and Julie decide to play a mindless prank on Liz the morning of her 17th birthday, by performing a fake kidnapping. They surprise Liz in bed, bind her with ropes, and Courtney rams a jawbreaker into her mouth to gag her, before sealing her mouth with duct tape. The girls then lock Liz in the trunk of a car and drive off, actually planning to take her to a restaurant for breakfast. Upon opening the trunk, however, they are greeted with the grisly sight of Liz dead, having choked on the jawbreaker.
Julie wants to go to the police, but Courtney forbids her. Courtney calls the school pretending to be Liz's mother and tells them Liz is ill and cannot attend school, then the three go to school as though nothing had happened. When school outcast and ardent admirer of Liz, Fern Mayo (Judy Greer), is sent by Principal Sherwood (Carol Kane) to deliver Liz's homework at the end of the day, she stumbles upon the three girls and Liz's mangled body. Out of jealousy, Courtney fabricates an elaborate story that Liz died at the hands of a rapist, and plots to tarnish Liz's good reputation by spreading false rumors that she was actually a rebellious, promiscuous girl, who drank and did drugs and was not the perfect angel she made herself out to be.
Fern attempts to flee the house, but the girls catch her and Courtney buys her silence by accepting her into the clique, telling her to take Liz's place, despite Julie's protests. Courtney and Marcie give Fern a makeover, transforming her from plain and awkward to elegant and beautiful. The transformation is so complete, Courtney introduces Fern as the beautiful exchange student "Vylette".
Julie, overwhelmed by guilt at her part in Liz's death, breaks away from the clique, only to be tormented by her former friends, and as her popularity dissolves, she becomes a new target for abuse and contempt throughout the school. Her only real friend during this time is her boyfriend, a drama student named Zack. As Vylette's popularity soars, Julie watches in silence as Courtney spins an endless web of lies to cover up the murder and maintain her popularity. Julie threatens to go to the police and tell them the truth, but Courtney retorts that she, Marcie, and now Vylette will claim Julie killed Liz if she attempts to expose them. To her disgust, Julie learns that, after they had returned Liz's corpse to her house, Courtney went out that same night and seduced a stranger (Marilyn Manson) at a sleazy bar and had sex with him in Liz's bed, making it seem as though he had raped Liz.
Vylette becomes intoxicated with her new-found popularity, which has eclipsed Courtney's own. Courtney orders Vylette to learn her place, but Vylette vows that if Courtney does not watch her step, then she will reveal the truth behind Liz's death. In response, Courtney and Marcie post enlarged yearbook photos of Fern Mayo all over the school with the message "Who is Vylette" written on them, revealing Vylette's true identity and leaving her humiliated by the entire school. Julie takes pity on Fern and forgives her for being corrupted by Courtney.
Feeling no remorse for the lives she has destroyed, the heartless Courtney attends the senior prom with jock Dane Sanders (Ethan Erickson). Meanwhile, Julie is at home going through a bag of Liz's belongings that were given to her. Upon finding a recordable greeting card she was fiddling with when Courtney was faking Liz's death scene, Julie discovers it has recorded Courtney's admission to the killing. Armed with this evidence, Julie, Fern and Zack hurry to the prom.
When Dane and Courtney are announced as Prom King and Queen, Zack sneaks backstage and broadcasts the card's message over the sound system. Disgusted, Dane quickly abandons Courtney while Marcie hides under a table. Horrified that her scheme has unraveled, Courtney tearfully races for the exit as the rest of the furious students pelt her with corsages and call her a murderer. Julie snaps a picture of her former friend's anguished face to immortalize the occasion. As Courtney's photo ends up in the yearbook, the film closes with one of Fern Mayo's quotes to Detective Vera Cruz: "This is high school, Detective Cruz. What is a friend, anyway?"

Question: Who does Courtney warn?
Answer: Vylette