Avalon High Coronation, book 3: Hunter's Moon
Meg Cabot et Jinky Coronado2009

Ellie has only one day left to get her boyfriend Will to truly believe he's the reincarnation of King Arthur, or the world will plunge into eternal darkness. But Will refuses to believe her! Meanwhile, the upcoming Homecoming game and dance have everyone distracted, especially Ellie. Some people aren't too happy about her Homecoming Queen nomination and are determined to cause trouble. Ellie's got to save her reputation and the world before the new moon on the night of Homecoming brings them all to a dangerous crossroad. If Ellie can't convince Will of his destiny, the world may never recover. . .