From "Unions Are Not the Problem" by Diane Ravitch
"I responded that I was puzzled. The unions don't seem to cause low performance in the wealthy suburban districts that surround our city. They don't seem to be a problem for the nations that regularly register high scores on international tests. If getting rid of the unions was the solution to the problem of low performance, then why, I asked him, do the southern states—where unions are weak or non-existent—continue to perform worse than states with strong unions? And how can we explain the strong union presence in Massachusetts, which is the nation's highest performing state on NAEP? I suggested that low performance must be caused by something else other than teachers' unions. I have not yet received a reply, so I suppose he is thinking about it.
It actually doesn't seem to be all that hard to get rid of incompetent teachers. It appears that 40 percent of all those who enter teaching are gone within five years, according to research that I have seen. In every district, to my knowledge, teachers do not gain due process rights for three years (in some places, it takes five). During those three to five years, their supervisors have plenty of time and opportunity to evaluate them and tell them to leave teaching."