Teaching is not a profession. Never has been, never will. A PROFESSION requires an extensive body of knowledge, not known to the general public, without which one cannot function.
Think about it: Doctor, Lawyer, Architect, Dentist...They all have a large body of knowledge that is unknown to the general public, and they must demonstrate a mastery of that body of knowledge in order to be certified to practice in the PROFESSION.
There is no existing or even possible body of knowledge that is required to teach. They are not even required to demonstrate a mastery of their SUBJECTS [through comprehensive testing].
Ain't no profession. Professionals don't join a labor union.