To be clear, I'm not saying that a math teacher needs to work as a mathematician, nor that a history teacher needs to work as a historian. If a would-be math teacher has 5 years experience in engineering, or a hard science, I would accept those as related fields. Similarly the would-be history teacher could work in politics.
Beckee, What you say is true that people who have success outside of teaching may want to stay there, or return there after trying teaching. Salaries may have to be raised to account for the fact that the bar has been raised in one way. On the other hand, Finland has a high bar for teachers, but their compensation isn't drastically higher than it is in the US, from my understanding (by many accounts, it's lower). Also, there are perks to teaching regarding summer vacations, and job satisfaction that aren't found in other industries.
The point is to let the market assess the understanding these teachers gained in the subjects they studied instead of developing subject specific tests for all teachers, while giving the teachers experiences they can relate to their students. Did you ever ask your math teacher the usefulness of understanding imaginary numbers? Could they answer? Mine couldn't. I think he said they were "used in engineering" but had no idea how, and could offer no example. He was generally a good teacher, but my motivation suffered because of his ignorance. (Luckily I didn't need much motivation to grasp math.)
Since you're a teacher with at least some outside experience, perhaps you can comment on whether or not you relate those experiences to your students in a way that enriches their education. My sister-in-law relates my engineering experiences to her math students because she has none of her own (she'd be perfectly capable of acquiring some.)
