Mr Majeika, Paperback/Humphrey Carpenter

Mr Majeika can behave just like any ordinary teacher if he wants to, but something has to be done about Hamish Bigmore, the class nuisance, and so he uses a little magic to turn him into a frog. And to everyone's delight it looks as if Hamish will have to remain a frog because Mr Majeika can't remember the spell to turn him back again.