A Great and Terrible Beauty (The Gemma Doyle Trilogy) by Libba Bray
This book wasn't really appropriate for younger readers. There was some swearing, some sexual references, the main girls drank, one of the girls cut herself, they talked about same sex relations, and there was brief nudity. The story was well written and the background interesting, but there were just too many unnecessary elements.
Gemma Doyle always dreamed of moving to England and being with people her own age. But when she watched her mother die by her own hand, she learned that getting what you wish for is not always a good thing. Once in England, Gemma was enrolled at the finishing school her mother once attended, and was forced to live with mean vindictive girls. Haunted by her mothers death, Gemma started experiencing strange visions that further alienated her from her peers. Scandals and secrecy finally united Gemma with a small group of girls, and together they formed The Order. The Order dabbled in magic and the other realms, and a costly mistake released evil into the world. Gemma must find the strength to forgive her mother and restore order, before Circe takes over the world.