One of Us is Lying  By Karen M. McManus

The story starts out like a modern day Breakfast Club, with five students stuck in detention. We meet Bronwyn, the Yale bound non-rule breaking "brain"; Addy, the picture-perfect homecoming queen "beauty"; Nate, the "criminal" already on probation for dealing; Cooper, the all star baseball pitcher athlete, aka "jock", and Simon, the "outcast" and creator of Bayview High's notorious gossip app. But then the story takes a dark turn. Before the end of detention Simon is dead.