It is about a young girl who goes to live with her grandparents. She has to go to a new school, which is a witch school where they fly on brooms - and the ones who have money fly on hoovers. To make their brooms fly they use a flower called fly-by-night. The girl finds that the school has been experimenting on animals, and that they have been turned into horrible deformed creatures. She rescues them and does a spell to turn them all back to normal. That's all I remember. Also I think the girl was called Mary Smith?