Frogspell by C.J. Busby (author)

Max Pendragon is probably the worst squire in King Arthur’s kingdom – he’d much rather train to be a wizard instead. So when he accidentally invents a spell that turns people into frogs, it seems his wish might come true.

But events take an unexpected turn when Max enters the Novices’ Spell-Making Competition and finds himself helping Merlin fight an evil sorceress. There’s no way Max’s unpredictable frogspell could be of any use. Or is there?