The worst teachers my sons had over the years were dinosaurs. I used to live next door to one of them and all she did was crab about how tired she was and how much she was sick of teaching after thirty years -- but there she was, year after year, destroying any interest her students might have had in her subject with her rotten attitude.
One guy just simply refused to follow any guidelines the district set for curriculum and his behavior in the classroom. My son used to come home complaining how the guy spent half of every class preaching his religion to them. Parents complained and there was nothing that could be done about it. Another old timer used to fall asleep on his desk while the kids were working or he'd ramble on an on (I watched him) about his pet interest that had nothing whatsoever to do with the subject he taught. Being in his class was like visiting Great Grandpa at the home, and yet again, there was nothing to be done about it.
Meanwhile, the young, bright energetic teachers, those that could best relate to their students and were the most current in their understanding of the subject matter, were the first to be laid off or moved around. Every year it seemed one kid or the other would come home bummed out or kids would be crying at school because Ms. Jones, who everyone loved, was being sent to another school while Mr. Smith, who everyone despised was taking over her classes.
The dinosaurs also had all the various extracurricular activities under their control because running a program meant an extra couple grand a year. And then they would just phone it in -- the same old stupid everything that the kids had lost interest in a decade ago. I know there are exceptions, good inspiring high seniority teachers, but they would do just fine in a merit system along with the best and brightest of new teachers. It's absolutely stunning that teachers' unions will claim anything matter more than performance.
It's like the schools are run primarily to maintain the jobs of the teachers with the highest seniority -- that's the prime directive -- not the best education for the kids or the best value for the tax payer. The entire system revolved around the teachers with seniority, like schools exist only to keep them comfortably employed.