One of Us is Next by Karen McManus (Teen)

A year after the Bayview four were cleared of Simon Kelleher's death a ton of copycat gossip apps have popped up, but no one's been able to fill the gossip void quite like he could. Now a game with dangerous consequences is targeting students at Bayview-- and if the creator isn't found soon, dangerous could prove deadly. If you choose not to play, it's a truth. And Phoebe is the first target. Maeve is next. By the time Knox is tagged, the dares have become deadly.