One of Us Is Lying by Karen M. McManus
Five classmates enter detention, but only four make it out alive. When police officers discover Simon's body and rule that it was anything but an accident, all eyes are on his four detention buddies. After all, Simon did threaten to post their scandalous secrets online. What really happened on that Monday afternoon? This is an addictive and thrilling YA read that explores how far we're willing to go to protect our secrets.