If I were your principal and we were setting goals for next year, what would they be?

You may want to ask for more information to make a more well-informed response. Regardless, you can answer hypotheticals by laying out the grounds of two different common scenarios and explaining the goals. You can also wrap your answer up by telling the interviewer about one aspect of professional development that you would strive to work on regardless of the school's needs. Follow up with why you want to work on it and/or how it'll benefit the school.