Through grammatical analysis, the answer came out. "Service" is the verb as the predicate, "teacher" is the noun as the object, and "mathematics" is the teacher's attribute. If you shorten this sentence again, it is: as a teacher. Obviously, being a teacher is incomparable with being a teacher. Teachers can "be" or "be". If you change the original sentence to this, it will not be a sick sentence: be a math teacher.