The Wind Singer by William Nicholson (The Wind on Fire Trilogy #1)

I hate school! I hate ratings! I won't reach higher! I won't strive harder! I won't make tomorrow better than today! Pongo to exams!
In the walled city state of Aramanth, exams are everything. When Kestrel Hath dares to rebel, the Chief Examiner humiliates her father and sentences the whole family to the harshest punishment. Desperate to save them, Kestrel learns the secret of the wind singer, and she and her twin brother, Bowman, set out on a terrifying journey to the true source of evil that grips Aramanth.