Jawbreaker (1999)

In what was meant as a harmless birthday prank, three of Reagan High School's most popular girls, Julie, Foxy, and Courtney pretend to kidnap their friend, the latter shoving a jawbreaker into the victim's mouth to keep her from screaming. Their plan goes awry when the girl accidently swallows the jawbreaker, choking to death. The cool and calculating Courtney tries to cover the crime but is found out by school geek Fern Mayo. In return for her silence, Courtney transforms the gawky Fern into the stylishly beautiful Vylette, leaving the conscience-stricken Julie out in the cold, threatening to set her up for the girl's murder if she breaks her silence.