The lying game / Ruth Ware.

The text message arrives in the small hours of the night: I NEED YOU. Isa drops everything, takes her baby daughter and heads straight to Salten. She spent the most significant days of her life at boarding school on the marshes there - days that still cast their shadow over her. At school, Isa and her three best friends used to play the Lying Game. They competed to convince people of the most outrageous stories. Now, after seventeen years of secrets, something terrible has been found on the beach. Something that will force Isa to confront her past, together with the three women she hasn't seen for years. Theirs is no cosy reunion; Salten isn't a safe place for them, not after what they did. It's time for them to get their story straight...