My Headteacher is a Vampire Rat - Pamela Butchart

Izzy and her friends plunge into more primary-school craziness. This time, they decide that their new head teacher is a vampire rat, based on his being slightly scary, having the blinds drawn in his office during the day and the fact he's banned garlic bread at lunchtimes. Now they just have to come up with a plan to vanquish him.