The Witch’s Key by Sarra Cannon

After the tragic death of her parents, seventeen-year-old witch, Lenora “Lenny” Thorne, is sure life will never be good again. It’s bad enough when The Witch’s Council sends her to live with her ancient Great-Uncle Martin, a man she hardly knows, but they also expect her to attend the local human high school and pretend to be a normal person.

To her surprise, though, Martin is actually amazing, and her first day at Newcastle High is better than she could have imagined. Maybe there’s a future for her here, after all.
But when one of her new friends goes missing, Lenny is pulled into an investigation that even The Council won’t touch. She’s in way over her head, but the lives of five local teens are on the line. She refuses to just let them die.

With the help of a handsome and mysterious new ally named Kai, Lenny vows to find the girls and bring the demon who kidnapped them to justice. Will she save her friend in time, or will she be the demon’s next target?