Oligarchy: Scarlett Thomas

When Tash, daughter of a Russian oligarch, is sent to an English boarding school, she is new to the strange rituals of the girls there. Theirs is a world of strict pecking orders, eating disorders angst.
While she spends her time at the lake and the stables, a few hand-picked girls are invited to join the Headmaster at his house for extra lessons. Then her friend Bianca mysteriously vanishes, and the routines of her dormmates seem darker and more alien than ever before.