Last month, 500 angry schoolteachers assembled outside my office. The United Federation of Teachers was furious that “Stupid in America,” a “20/20” show I did on education, suggested that some union teachers were lazy. They shouted that I didn’t understand how difficult teaching was, and chanted, “Shame on you!”

Randi Weingarten, head of New York City’s union, took the microphone and hollered, “Just teach for a week!” She said I could select from many schools. “We got high schools, we got elementary schools, we got junior high schools!”

I accepted. I even said I’d let the union pick the school. I thought I’d learn more about how difficult teaching is. Above all, it was a chance to get our cameras into schools – something the N.Y. bureaucracy had forbidden – so we could show you what was really going on.

But it won’t happen.

Like most of our dealings with the union, nothing was easy. It took weeks of phone calls to make any sort of progress. I suspect this will not surprise public-school parents.

Finally, the union picked a school: Beacon High. Unfortunately, it’s not a typical public school – it’s “special.” Beacon doesn’t have the full incentives or flexibility of a private school: It can’t go out of business, and it is burdened by bureaucratic rules and a union contract. But Beacon offers a limited form of what the union opposes: school choice. As with a private school, you don’t have to go there, and they don’t have to take you. Applicants must submit portfolios, and if too few chose Beacon, it wouldn’t be able to remain special. To remain what it is, it must compete.

Recently classes of Beacon students took field trips to France, South Africa, and tellingly, Venezuela and Cuba. Beacon has rooms filled with computers, students learn to do Power-Point demonstrations, and a class I watched had two teachers (one a student-teacher) for 24 students. Ninety percent of Beacon’s students graduate, while the average graduation rate for New York City public schools is only 53 percent.

I guess they didn’t want me to look at a normal public school.

But this is the school the UFT picked, and I was up for the challenge. Who knows what I might have learned by teaching?

My producers went to a meeting at the school to choose a class for me to teach. The union representative didn’t come, so we were told no decisions could be made. Lots of people came to a second meeting at the school: four people from the union, one person from the city Department of Education, and administrators and teachers from Beacon. They decided I might teach history classes and “media studies,” but they would have to talk to more people.

You would think my teaching had been my crazy idea. I was only trying to accept the union’s offer.

I prepped for my history classes. We had more meetings. The school principal had me sit in on a class with a “superstar” teacher. It was supposed to be a history class, but he seemed to teach “victimhood in racist America.” On the class door he posted a New York Times column denouncing the president for spending too much money on war. Can we say “left-wing”?

Then there were more meetings. Finally, four days before what was supposed to be my first day of class, they canceled. Officially, “they” were the public school administrators who said it might be “disruptive” and that it might “set a precedent” that would open their doors to other reporters.

Too bad. Letting cameras into schools would be a good thing. Taxpayers might finally get to see how more than $200,000 per classroom of their money was being spent.

I wonder why the union even made the challenge. I suspect the UFT didn’t expect me to say yes. When I turned out not to be easily intimidated, the teachers’ union and the government school monopoly folded. Perhaps there’s a lesson there.

But I wasn’t trying to call a bluff. I wanted to accept an invitation. I’d like 20/20’s cameras to see me struggle to be a good teacher.

I wonder what else our cameras might see.

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