Pennyroyal Academy by M.A. Larson
A girl with no name and no memory finds herself in the middle of a war. On one side, princesses and knights. On the other side, witches and dragons. After enlisting to become a princess, she is taken to Pennyroyal Academy, where she is given the name Evie and trained to battle the dark forces that are growing throughout the kingdoms. But the war is much more personal than she could possibly ever imagine, and as she learns about her past, she must make some very difficult choices that could affect her life and the lives of everyone she knows.