I have never had to have any dealings with the union. My experience (and I'm on my 12th year) is that if you have a good relationship with the principal and the counselor, and you all work together, you can arrange little pockets of sanity for the good kids.
Again, very good!
I know someone - very liberal - who lives in a divided town in a northeastern state. By "divided," I mean half the town is very well to do, and the other half is starkly low-income.
He was delighted when - just before his only child was to begin kindergarten - a "magnet school" opened on the low-income side of town. He was delighted because he lived on the high-income side, and he was concerned that if his son had gone to school in his nearby neighborhood school, he (the son) would have been robbed of the sort of diverse educational experience my friend desired for him.
Anyway, the "magnet school" didn't last long. The teachers' union worked with a local "advocacy organization" to demonstrate that a disproportionate number of the children admitted to the lovely magnet school were not "of color."
The parents didn't like the magnet school because too many of their children weren't admitted because they didn't score well on the required aptitude tests, and the teachers' union didn't like it because of the favoritism shown to those teachers who got the comparatively pleasant task of teaching the more highly-motivated students with attentive and involved parents.
So after a year or two, the whole enterprise collapsed in a melee of race-baiting and finger-pointing.
Around that time, my friend stopped being my friend, also.
I had expressed skepticism about the "magnet school" in the nasty neighborhood, but there may have been other factors involved. To this day, I don't know.