Hundreds of teachers are expected to gather outside the ABC television studios in Manhattan this afternoon to protest a news story the network aired in January that criticized teachers unions and called public schools a monopoly.

In a letter circulated to city educators, the United Federation of Teachers condemned the “20/20” segment called “Stupid in America” and asked union members to sign a petition and join the demonstration.

“It presented a biased and inaccurate picture of teachers and teacher unions, especially New York’s United Federation of Teachers, blaming them for all the shortcomings of American education,” the petition read.

The union is demanding a correction from ABC News and is asking the Disney-owned network to “enforce standards of journalism that ensure fair and accurate coverage of education issues.”

The segment, reported by a “20/20” co-anchor, John Stossel, aired on January 13.

The schools chancellor, Joel Klein, told Mr. Stossel that it’s “almost impossible” to fire a teacher in New York City.

“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,” he said.

Even after a teacher admitted to sending sexual e-mails to one of his 16-year-old students, Mr. Klein said he was unable to fire him. The chancellor said he was forced to keep the teacher on the payroll until he was finally fired after six years of litigation.

“He hasn’t taught, but we’ve had to pay him because that is what is required under the contract,” Mr. Klein said.

“Ah, yes, the union contract,” Mr. Stossel responded. “Here it is, more than 200 pages of fine print. Union monopolies often create documents like this. Everywhere, unions resist the practice that made GE and other organizations successful. Weed out the bad. The rules must stand, say unions everywhere.”

A spokeswoman for the Department of Education, Kelly Devers, declined to comment on the rally and referred back to the chancellor’s comments on the show.

The American Federation of Teachers, the parent union of the UFT, is helping to organize rallies and petition drives today in five cities, including Detroit and Los Angeles. It posted a 20-page response to the show on its Web site and distributed it to its members.

An avowed libertarian, Mr. Stossel is the author of “Give Me a Break: How I Exposed Hucksters, Cheats and Scam Artists and Became the Scourge of the Liberal Media.” He has received 19 Emmy Awards and the George Polk Award for Outstanding Local Reporting. His syndicated column appears in The New York Sun.

The president of the United Federation of Teachers, Randi Weingarten, is planning to hand ABC a petition with “thousands of signatures” today.

“ABC News should cease presenting agenda-driven reports that masquerade as balanced journalism,” Ms. Weingarten said. “And John Stossel should be held accountable for the inaccuracies and distortions in his reporting.”

A spokeswoman for ABC News, Alyssa Apple, said the show “raises important questions about our public school system and asks why so many American kids are being cheated out of a quality education.”

“The report covers not only problems within the system, but offers a look at programs that have been more successful. … Mr. Stossel welcomes criticism from the teachers who are taking time out of their day. Mr Stossel plans to continue to report on this important topic,” she said.

Ms. Weingarten also plans to read a protest letter sent to Disney’s CEO, Robert Iger, and signed by two dozen winners of Disney’s American Teachers Award.