Fae School — Charles G. Dyer

A terrible accident turns young Kerry into an orphan. Alone in the world, she is placed in her nasty uncle's care. He and his family make her life even more miserable until the lawyer arranges a change of guardianship.
The lawyer reveals that her deceased parents had kept life-changing secrets from her.
She is a fairy, but the only way to change into her true self is to go to a Fae school and undergo a transformation ritual.