Astra knows what she's good at: thieving, tricking, and hitting people with rocks. Especially the mayor's bullying son, after he makes fun of her. She also knows what she's not good at: being a girl. Not the kind of girl that lives in the tiny mountain village of Theornin, anyway.
So logically that means making Wizard Jorphen change her into a boy.
But the mayor's already pushing Wizard Jorphen around to do something else: stopping a mountain from growing under the village and pushing everyone out. Except the only way to stop a mountain from growing might kill Astra's only friend.