The Ill-Born Prince — C. Dale Brittain

Antonia thought she would grow up to be a small-town witch, like her mother. But then her father was made Master of the previously all-males wizards' school, and she became a student there. Prince Walther thought he would grow up to be the ruler of his kingdom. But then it was revealed that his real father was not the king but a renegade wizard, and he too became a student of the magical arts. And now, just when both are trying to come to terms with their parentage while negotiating their relationship to each other, the dragons show up -- not to mention those who would overturn the whole structure of institutionalized wizardry. At this rate how will they ever get back to class?