One of Us is Lying by Karen M. McManus
Five students are sent to after-school detention at Bayview High: Bronwyn, the brain; Addy, the beauty; Nate, the criminal; Cooper, the athlete; and Simon, the outcast. You might think this sounds like a familiar '80s movie about a club and breakfast—but in this case, Simon doesn't make it out alive. Simon, the creator of Bayview High's infamous gossip app, had promised to release juicy details about all four of his popular co-detention attendees the next day, which means all four are suspects in his murder—unless, of course, a sixth figure is using them as patsies.
Review from School Library Journal:
A smart, twisted, and unpredictable YA mystery that will have readers guessing until the very end. This debut novel follows four high school students thrown together in detention with the creator of their school's gossip app. All of them come out alive. The creator doesn't. And now they are all suspects in his murder. Each student takes a turn at narrating, and tension builds as secrets are revealed in a way that mirrors the gossip app. Who is telling the truth? Who is lying? McManus uses alternating viewpoints to tell the story. Each chapter is short and quick, which makes the plot move at a fast pace and will engage readers. At the beginning, keeping characters straight might be a tough task for some, but it is easy to understand the motives of each character, and they soon become easier to distinguish. This factor makes it hard to know who is guilty and who is innocent.