This question has spawned many answers, not always good ones. Two weeks ago I wrote about the experience I and Nancy Richards, our Director of Junior School, were embarking on in Toronto, as members of an accreditation team. The goal of these accreditations is to make good schools better. Since different schools have different missions, you can count on some variation in that path to improvement.
At our school not only do we regularly ask this question, we constantly ask it. The answer, we find, usually involves applying some new layers over enduring principles. Most often for us the enduring principle part tends to be excellent teaching, whereas the new layer part tends to be something like current brain research, personalization, or professional growth.
Above is a Ted Talk on the subject, well-researched and thought-provoking.