Sticks & Stones (Upside-Down Magic, #2)

Nory, Bax, and their friends are in a special class at Dunwiddle Magic School, a class for children whose magic is unpredictable--and while Nory is finding an outlet for her dritten-self (a combination of a kitten and dragon), in the kittenball team, Bax is worried because all he can do is turn into a rock, and nobody knows why.