The Librarian's Bookshelf - Liar -- Justine Larbalestier

The story starts when the principal announces that Zach has been killed. Everyone is stunned, and then one of the students suggests that Micah, Zach's "after-hours" girlfriend, was his killer. The story behind the narrative is actually rather ... Her alibi is, depending on whether you choose to read the novel as realistic fiction or delusion/lie-induced fantasy, either completely convincing or utter crap. It's impossible to know whether Micah's story is true or not, ...