It's very easy to point the finger at unions and the difficulty of being fired, but when you look closer at the issue it's not so simple.
As the child of 2 teachers, I hear stories from my parents all the time about the horrors of the teaching system.
In my mother's elementary school, the parents regularly threaten to sue the school board over the grades that their supposedly perfect children are not receiving on homework. The board caves every time a lawsuit threat is filed. I can't even begin to imagine what would happen if the teachers themselves were easier to fire. You'd have great teachers being sued by parents and losing their jobs all the time.
My father's high school is a robotics teacher one of the leading edge tech schools in the city, with over 20 world place finishes in these competitions. Recently, he came under fire from his principal because he wasn't willing to play along with her personal ambitions that were detrimental to the student's education. If it wasn't for his union rights, he would have lost his job over a matter of politics and an unethical principal.
I've had more than my fair share of poor teachers, and I do wish that they could be encouraged to quit. However, I think that stripping union rights would be a case of throwing the baby out with the bath water. There's many better ways to encourage good teaching, such as through positive reinforcement systems.