I remember a chapter book for middle school aged kids, I don't remember much about it but what I do remember is a girl attends a magic boarding school, I think there is a shapeshifter involved, like a student or something. All I remember is at the end there are multiple dimensions and the version of herself in that dimension dies but another lives? And there is some evil involved. The cover art is yellow, with a brown haired girl and bubbles all around her that contain images. Please help me, I can't for the life of me remember what this book is called!!
It was definitely English, either American or British.