My daughter and her hubby are both teachers, and it took a university degree plus further stuff to get to teaching a class; I'm not sure how the unions function, but they can't have done very well as the salaries are pretty low... However, I do think I was told that some reach the teacher level without a degree and need another sort of qualification. Perhaps that limits the harm they can do higher up the grade because they work lower down the totem pole. However, bad teaching at any level will be paid for by the pupil.
If there's a problem in schools today (I only speak about Scotland) then I think it's a matter of discipline. When I was a schoolboy it never entered out heads to treat teachers with disrespect; they would never have tollerated such a dumb thing. Further, going home with a bad report or with a telephone call from the school arriving home before one, would have resulted in a further heap of troubles! Come to think of it, I don't think anybody I knew was inclined to be rude to adults; I don't believe it entered our heads. Why would it?
Today, some teachers fear the parent/teacher interface because of violent parents!
The blame for that, in my mind, falls squarely on the heads of the morons that took over basic education during and after the 60s. Worse, the same tribe seems to have taken over all the posts in the judiciary, too. Nobody gets legally thumped for anything. Unless it involves money. Steal that in a big way and you will be chased forever. Unless you are a Member of Parliament, when they will close ranks about you and obfuscate and stonewall until the press forgets.