Isn't this basically a tenure thing? AFAIK public unions make firing a First-In-Last-Out operation, which means the senior teacher you hired 10 years ago is there to stay until every single other teacher that came after is gone.

In my completely non-expert opinion, tenure seems to be a multiplier. From a students' standpoint: A good teacher with tenure is a godsend, because they feel comfortable challenging the system to make sure their job (educating the kids) gets done. Conversely, a bad teacher with tenure is a cataclysm, because they don't really want to be there, the kids don't really want them to be there, and the school administration doesn't really want them to be there, but by God, they're staying, because it's easier to collect a paycheck and hate your job than change your situation. To use a metaphor from Kevin Smith's Clerks, tenured crappy teachers are perennially shitting their pants instead of lifting the lid on the toilet seat.