You can come up with a list of bad things that might happen if teachers didn't have unions but it only only takes a tiny bit of research to see that laundry list of bad things that are happening because of them. I am ant-teacher's union but pro-teacher. My wife and I both volunteer at my daughter's school, we help purchase supplies for her classroom, the teacher even does things like send home 35 pencil and a sharpener and ask to have 35 sharpened pencils returned the next day. I love my daughter's teacher and I think she should be rewarded for the excellent job she does teaching my child and not simply because she has been there more than two years. She should be paid better and have better job security than less qualified and motivated teachers.

And sorry I believe the majority of schools rate teachers are how well students advance and not on how well they did. If you are teaching 4th grade and students come in reading reading at a second grade level then you are evaluated on how much those students advance compared to other students in other classroom at the school. So if they leave at the third grade level you did OK, if they leave at the fourth grade level did an excellent job and should be rewarded but if they leave still reading at the second grade level you should be evaluated and either change very quickly or be fired without the school needing to hire a lawyer or pay you for five years while they wait for hearings. And it should be quick, you shouldn't be able to be a piss poor teacher for hundreds of students before they replace you. If the principle doesn't think you are doing the job he or she should be able to replace you in the middle of the school year. Just like in the real world.

Some teachers consistently have students fall behind in their classrooms and the school can do nothing because of unions. One of the best things they could to improve schools is empower principles to promote good teachers and fire bad ones then fire principles who can't improve their schools. The removal of accountability is one off the reasons government is so dysfunctional. Schools are set up to educate students and that not protecting jobs for adults needs to the priority.