The Charmed Children of Rookskill Castle by Janet Fox
Something is not right at Rookskill Castle, a rundown Scottish manor shrouded in mystery. The castle is a temporary boarding school for children escaping the Blitz, but soon it’s clear there is something terribly wrong. There are clues hinting that a spy is in the house, and there are undeniable signs of a sinister magic. When the children in the castle’s temporary boarding school begin disappearing one by one, it’s a race against the clock for twelve-year-old Kat Bateson, her two younger siblings, and their new best friend.