“The teachers united will never be defeated!” chanted thousands of public-school teachers at a union rally. They may be right – unfortunately. Teachers unions in this country are very influential because they can assemble a crowd. Randi Weingarten, head of New York’s teachers union, put out the word, and thousands of teachers filled Madison Square Garden to demand a new contract and more money. That clout brings timid politicians into line.

The unions can pay for expensive rallies at “the world’s most famous arena” because every teacher in a unionized district like New York must give up some of his salary to the union. Even teachers who don’t like the union, teachers who believe in school choice, and teachers who could make more on the open market must fork over their money to support the unions that fight against school choice and merit pay.

The unions use their clout to fight against the interests of the best teachers. Union leaders make sure the teachers who work hardest don’t get raises or bonuses. Everyone with the same seniority and credentials must be paid the same.That guarantees that no teacher will take home a dime for making extra sure that students learn. Joel Klein, who as New York’s schools chancellor runs the country’s largest public-school system, put it this way: “We tolerate mediocrity, and people get paid the same whether they’re outstanding or whether they’re average or, indeed, whether they’re way below average.”

Klein said that out of 80,000 teachers, only two have been fired for incompetence in the past two years. That’s because it takes years for a principal to fire an incompetent teacher. I can’t explain the rules here, but you may be able to read a flow chart about them in my next book – “may be” because the flow chart may be too big to fit in a book. The rules are so complex that they ought to begin: First, take a week off from running your school to study these rules. Many of the rules come from the union contract, which has 200 pages plus a mess of addenda. Even Klein, who used to practice antitrust law for the federal government, called the contract a “regulatory nightmare.”

But the unions fight to protect the nightmare. Weingarten has a remarkable excuse: “Our union has actually stepped up to the plate and said we’ll police our own profession.”

I’d like to police my own job, too. And I’ll bet some students would just love to police their own homework!

Of course, unions do more than just protect incompetents. Weingarten, on behalf of New York’s teachers union, fought for a uniform day of six hours, 40 minutes. “Which is what normally happens in the private sector,” she told me.

Funny. I work in the private sector every day, and I haven’t seen that. Have you?

The teachers no longer have that either, though. Last year, they made a big concession. Now they have a uniform day of six hours, 50 minutes. That’s nearly a whole additional hour every week!

Some teachers care about the students, so they want to do more than the contract requires. But astoundingly, some of them told me they are actually afraid to stay at school when the union says it’s time to go home. They worry they’ll “get in trouble with the union.” It’s as if the teachers, united, never to be defeated, made a decision: Instead of letting the administrators crack down on bad teachers, the union will protect the bad teachers by cracking down on the good ones.

Maybe that’s what Weingarten calls policing their own profession.

I confronted Weingarten. “Unionized monopolies like yours fail. In this case, it is the children who – who you are failing.”

“We are not a unionized monopoly,” she retorted. “And ultimately those folks who want to say this all the time, they don’t really care about kids.”

Really, Ms. Weingarten? You fight to protect a system that rewards mediocrity, and then you claim your critics don’t care about kids?

Give Me a Break.

Mr. Stossel is co-anchor of ABC News’s “20/20” and the author of “Give Me a Break.”