Teeth (2007)

Dawn grows up in the shadow of a nuclear power plant. In high school, while her biology class studies evolution, she realizes she may have a hidden curse, an "adaptation." She lives with her mom, step-father, and hard-edged step-brother. She likes Tobey, a guy at school, and he likes her. She takes a pledge to remain chaste until marriage, so they date in groups, watch G-rated films, and don't kiss, but the power of teen hormones is great, so temptation beckons. Dawn has an admirer in Ryan, and when when things have an unexpected twist with Tobey, she turns to Ryan for help. Will he be her mythical hero and rescue her? Or can she find her way as her own hero, turning the curse into an asset?