My mom is on the school board in my district and we frequently discuss the incredible harm inflicted on our public education system by teachers' unions. The thing is, I know a good number of teachers that I like personally, but the socio-political structure of the various unions is focused on one thing: not educating kids, but enriching and protecting teachers. Which is what you'd expect! Everyone is out for their own good. The problem is that our public education system as it stands now doesn't create an incentive structure to harness that innate selfishness and use it to benefit our kids.
There are two ways to redirect selfishness. The first is religion, or any other system that promises deferred, intangible rewards in exchange for surrendering present self-enrichment (like communism). This approach works very well if you can get everyone to play along by the same rules; unfortunately(?), history shows that you can't force a whole society to conform, and religion creates an unstable equilibrium wherein those who cheat the system can reap all the benefits without paying any of the costs. A system can only bear so many "free-riders" before it begins to break down.
The second way to redirect selfishness tends to be far more stable and better able to resist cheaters: competition, the foundation of capitalism's prosperity. Unlike religion, competition doesn't reward free-riders because in a purely competitive system (which exists only in theory) there are no free-riders. Each person earns the rewards of his own labor, and no one is forced or coerced into supporting his cheating neighbor. Competition leads to a stable equilibrium because cheaters are removed from the system and don't drain resources from those who choose to participate.
The reason, then, why teachers' unions are harmful to children is because their whole goal is to eliminate competition among teachers. Unions are, to the best of my knowledge, uniformly opposed to merit pay and insist that teachers' salaries be tied to seniority rather than performance. Unions are also against school vouchers or any other system that would allow parents to decide where their child goes to school. If parents are allowed to direct the "public" money that pays for their childrens' education (it's not really "public" money, it's their tax dollars) then teachers' performance will be implicitly evaluated by parents and reflected in their school choices. As Ed at Captain's Quarters explains about the National Education Association:
The efforts at educational reform have unnerved union leaders due to the [Bush] administration's determination to hold schools responsible for their performance -- a philosophy that threatens to undermine the ridiculous "tenure" model that makes removal of ineffective teachers an almost impossible task.The only way to truly reform our education system is to eliminate the socialist model currently in place and to introduce at least rudimentary competition that will weed out the incompetents.
But what they truly fear is an effort to implement a school-voucher plan that would for the first time create a competitive market for educating the children of working families instead of just the richest families in America. Competition would either force public schools to reform themselves and their evaluation processes or face obsolescence. Good teachers, of course, could find work in a boom of private-school openings that vouchers would create or negotiate better conditions for themselves at the public schools that would want to hang onto them. The effect of the NEA's opposition to change is to protect the least competent among them, a fact not lost on several teachers I know personally.