Translation:He is the school's most popular teacher.
Why is "teacher" in the indefinite rather than in the definite form, i.e., "Han er skolens mest populære læreren"
So if there is indefinite form, why is there "populaære" instead of "populær"?
English only inserts the definite article when the word order is reversed: "He is the school's best teacher" or "he is the best teacher of the school." Either way, the only true definite here is 'school'.