Eon: Rise of the Dragoneye Alison Goodman

Twelve-year old Eon has been studying the ancient art of the Dragoneyes for two years. But he is playing a dangerous game: Eon is actually Eona, 16 years old and a girl. Her true identity must remain hidden at all costs: it is forbidden for women to practise the Art, and to be discovered would be punishable by death.