Here's an excerpt from an interview with Diane Ravitch on How Teachers Unions Lead the Way to Better Schools:
What do you see as the role of teachers unions in preserving public education?
For many years, there has been an effort to diminish teachers unions and to blame them for all the problems of public education. I believe the reason, first of all, is that some people just hate unions. But there’s also a political reason that’s very specific. That is that if you silence the union, then there’s nobody at the table when the legislature or the governor wants to cut the budget, so they can hack away at will. That’s happening in states across the country. I was in Texas a few weeks ago, and there the legislature cut over $5 billion dollars from the education budget, but they did manage to squeeze out $500 million dollars for more testing. They have a weak union. They had no one at the table to say, “You can’t do this.” And no one cared what the teachers thought anyway.