Lunatics and Luck - Marcus Sedgwick

Meet Valevine the inventor, Minty who was once a witch, gorgeous, glum Solstice, her little brother Cudweed and his monkey, Fellah, and Edgar the raven, their self-appointed Guardian.

Full moons and money troubles are nothing out of the ordinary at Castle Otherhand. But add a horrible, hairy, howling new school teacher, complete with mysterious heavy wooden trunk, and the earth trembles and the body count rises.

Solstice and Cudweed are at his mercy. Gasp!