(1) Everyone in your class gets an "A" on a major test or (2) Not one discipline problem in your class for a whole month.

It would be great to say "both!" But the principal is looking for you to demonstrate your thinking here. First, recognize there are no "wrong" answers to this type of question. Look at this as an opportunity to show some rational and creative thinking.

You might focus on how one option involves a "positive" (all get a good grade) while the other option involves a "negative" (no one is "bad"). Since you are more of a positive-oriented person (the glass is half full) you would go for the good grades. Your goal as a teacher is for all your students to understand what you are teaching. A discipline problem now and then is a small price to pay for this understanding.

On the other hand, you might indicate that a class without discipline problems is reflective of a group of students who not only respect you as a teacher, but also each other. Your ability as an educator to teach your students this respect is, in many ways, just as important as teaching them history or spelling.