I think teaching is a talent that can’t be learned in college. That means teaching effectiveness can’t be improved greatly with degrees, particularly advanced degrees.
Everyone has encountered some PhDers who can’t communicate basic ideas.
Back in the 19th century, as depicted in contemporary literature like the "Little House on the Prairie" and "Anne of Green Gables", elementary-school teachers were generally young women. Lucy Maud Montgomery, for example, author of the Anne of Green Gables series, worked as a teacher. She got her teaching license after finishing a one year program in college (which was geared for two years, but I guess she was bright and energetic).