"You're suggesting that teachers basically be hung out for anybody to take a potshot at without any independent support at all. Some teachers are not good at their jobs, and some teachers exhibit bad judgment sometimes. Some parents are real pips too. Without due process and independent review, in which unions play a role, teachers who are doing their jobs are at their mercy, such as it is."

I'm suggesting that unions improve the teaching profession. You seem to see the situation as teachers vs parents.
How is listening to parents (and doing "something about it"- otherwise parents won't waste their time saying anything) leaving teachers "at their mercy"? You seem to be implying the union would neccessarily be incompetent. You are suggesting that the union although depending on fianancial support from teachers will have a bias against teachers.