The Broken Girls by Simone St. James

Vermont, 1950. There's a place for the girls whom no one wants - the troublemakers, the illegitimate, the too smart for their own good. It's called Idlewild Hall. And in the small town where it's located, there are rumors that the boarding school is haunted. Four roommates bond over their whispered fears, their budding friendship blossoming - until one of them mysteriously disappears....
Vermont, 2014. As much as she's tried, journalist Fiona Sheridan cannot stop revisiting the events surrounding her older sister's death. Twenty years ago, her body was found lying in the overgrown fields near the ruins of Idlewild Hall. And though her sister's boyfriend was tried and convicted of murder, Fiona can't shake the suspicion that something was never right about the case.