The Reunion by M. J. Arlidge

A skull looks up at Jennie from the trench, but it’s not the chalk-white bone and grimacing teeth that send her reeling. It’s the heart-shaped gold pendant, its delicate chain snapped in two. The necklace Hannah never took off. It can’t be Hannah. But it is.

When Jennie Whitmore arrives at her school reunion, she immediately regrets her decision. Why would she choose to surround herself with people who were never nice to her? Who still aren’t, even now she’s a police officer? The only person who truly looked out for her all those years ago was charming, beautiful Hannah. Until the day she disappeared.
Jennie is ready to finally put White Cross Academy behind her, the old school building demolished the morning after the party. But with the demolition comes a call: a teenage girl’s remains have been found on the grounds.
The instant drop in Jennie’s gut tells her that the remains might be Hannah’s, but when she’s called in to examine them, the truth becomes undeniable. Hannah didn’t run away and abandon Jennie thirty years ago; in fact, she never left White Cross at all.
Suddenly, Jennie has a murder to solve. The murder of her best friend. But can she do so before her colleagues discover just how closely connected she is to the victim? Before a mystery stalker makes good on his threats to silence her for good?