I don't agree with you on this. There have been too many unqualified teachers instructing, for example, math.
Maybe in the lower grades, but not once they get to middle school or higher.
I am wondering if my own kids were ever taught what a noun is. They both write pretty well, but I doubt they were ever shown how to diagram a sentence.
I realize I'm dating myself with that.
I'm talking about the primary grades. Junior High is early enough for the classroom shuffle. When students are ready for higher mathematics and other subjects, then it is a good idea for the teacher to hold a specific degree in that subject.
I agree that there are many unqualified teachers, especially in math. You have to also consider that the younger crop of teachers have been edukatud in publik skools. I've seen the papers that have been graded that my youngest one does. The teacher missed correcting misspelled words. I spend time with her, but I have to wonder what she's doing for 6 hours a day, to warrant the amount of homework she gets.