The Poison Apples by Lily Archer
We're reclaiming the apple! We all have evil stepmothers and we're going to give the apple back to them!
Alice, Reena and Molly all have one thing in common: they are the stepdaughters of three very evil stepmothers. They think they are alone in their unhappiness until they meet at Putnam Mount McKinsey, a posh boarding school located in lovely, rural Massachusetts, where Reena comes up with a cunning plan to exact their revenge . . .