My Mum's from Planet Pluto by Gwyneth Rees
Daniel knows that his mum was very ill once - mentally ill. She's fine now, and she's even landed a big new job as a head teacher. The problem is, she's head of his new school! It's so embarrassing - everyone is going to tease him.
But then things get even tougher for Daniel. With his father abroad, his mum stops taking the medication that prevents her illness coming back. As she starts behaving more and more weirdly, Daniel realizes that something is terribly wrong. And that he is the only person who can help her. . .