It's really simple.
We don't want to select teachers by seniority but by quality.
Basically unions try to protect the rights of the old majority or that's what it looks like if you are in the wrong demographic.
My mom was a teacher & got layed of over a seniority issue or rather she would get positions but they would be temporary, replacing burnt out incompetents with a job garantee mostly. She's a baby boomer but only started working as a teacher after another career.
Her sister had a similar problem because she had been a stay at home mom for a long time.
I heard them discussing these issues all through my childhood & still remember some specific acronyms related early eighties laws. Occasionally my mom will ask why i never went into education & i quote those then their main issues at the time.
Meanwhile in high school i had this physics teacher who was really not interested. He went through the motions but he was not inspired, not inspiring and not competent either.
He was less interested then us , a class of kids at the height of puberty. That was some kind of accomplishment. I always wondered he was still there but i figured he was sticking out his last years. Saw him smile once when a classmate of mine had handed in his 'cheating note' (a small not where you put all the formula's etc that you'll need) with the answers.
Then the school paper published a special edition because it existed 25 years and it had an article almost 20 years old with the same complaints about his lack of interest. It was hilarious & also awful.
We need inspiring teachers.
Also we need a clear goal of what we want to teach the kids because these days things are bureaucratic. Politicians make school budgets dependent on % graduated. Should be at least N% for decent funding and N% will graduate even if you have got to cut corners.
Test need to be tests, it's the only way it's worthwhile.
But that's another broader discussion (and i don't think the unions care to participate).