The Detained by Kristopher Triana
When Phoebe McBride returns to Bonneville for her twentieth high school reunion, she tells herself it's the best way to confront what has haunted her since her senior year. There was one boy who never lived to see graduation, and in a way she blames herself for this tragedy. Now a child psychologist, Phoebe is determined to face her demons by going back, but those demons may be fresher than she realizes.
When she arrives at the school there are only three of her old classmates present--a bad boy turned writer, a fallen football hero and a popular girl whose life isn't all she'd thought it would be. Bonneville isn't even set up for a party--it's set up for detention. Tables are aligned and her old P.E. teacher sits waiting. He always hosted detention back in her school days, but now he thinks he's here to accept an award for all his years of service.
Soon the guests discover gruesome keepsakes waiting on their chairs. Horror and paranoia sink their claws into the class of '96 as they are forced to revisit the worst memory from their youth, and ultimately pay for their past.