Island where kids mutate into monsters?

That's the main thing I remember about the book honestly. I think it was a series but I only ever read the first one. It all takes place on an island and I think a strange teacher shows up one day and towards the end of the book they all end up mutated in some way. One girl has wings and I think the main character became a manticore? There might have been a manticore on the cover as well