A Darkness of Dragons by S. A. Patrick Part of the Songs of Magic series

In a world of dragons, song spells, pipers and battles… Patch Brightwater is a boy in disgrace. Thrown in jail for playing a forbidden spell, he is no one’s idea of a hero. But then he discovers a deadly truth – the evil Piper of Hamelyn is on the loose. With the help of Wren, a girl cursed to live as a rat, and Barver, a fire-breathing dracogriff, Patch must stop the Piper sparking the biggest battle of them all. Three accidental heroes versus one legendary villain… an epic adventure is born.