Skeleton battle

"Overworld Academy has a new headmaster and the school is thriving. But the peace is broken, when one night a hoard of skeletons and hostile mobs attacks the grounds. Night after night, the mobs return, and the school is under siege. Is Isaac, the school's old evil headmaster behind the attacks? Or is it possible the new headmaster isn't what she seems--could she be working with him? Lucy and her friends are determined to get to the bottom of the mystery and keep their school safe. Meanwhile, the school is gearing up for an end of the year celebration, where everyone will showcase their greatest skills--from master building and clever potion-making to strategic fighting. As the mystery unravels and things come to a head, Lucy, Jane, and Phoebe come up with a plan to take care of the evil infiltrating the Academy, but there is no room for error in this battle against time"--Amazon.com.