Sue from West Virginia wanted to know: How do you evaluate teachers fairly in a merit-pay system? Cynthia from Minnesota asked what's next for the No Child Left Behind law. And teachers from Baltimore and Boston asked why charter schools seem to get special treatment.

To them, and about 2,000 others gathered at a Washington hotel yesterday for the American Federation of Teachers conference, U.S. Education Secretary Arne Duncan offered variations on the same answer: Trust us; we'll work with you.

But in his unusual town hall-style appearance before the teachers' group, Duncan urged the union to join the Obama administration's push to build support for a new wave of school reform as Congress prepares to reauthorize the 2002 No Child Left Behind law.

Merit pay, also known as "pay for performance," typically involves linking teachers' salaries to student achievement. It is probably the most sensitive issue that President Obama has raised with teachers' unions that campaigned for his election last year.

"You have to do this with teachers. You have to bargain it with the union. You can't do it unilaterally. That's the first thing," Duncan said.

And, he said, "don't pit teachers, professionals, professors, against each other."

On charter schools, which are independently operated but publicly funded, Duncan said: "I don't want them to get a free pass. . . . What you're asking for is fairness, for equity, and I couldn't agree with you more." He added: "To be clear, I'm not a fan of charters. I'm a fan of good charters."

Duncan and Obama have pressed repeatedly this year for states to ease policies that limit the growth of charter schools. That, too, is a hot button for teachers, because many charter schools are not unionized.

As he sat on a stool yesterday next to federation President Randi Weingarten, Duncan, in a gesture of solidarity, wore a button the union had distributed to the audience: "With us, not to us."

The slogan alluded to pledges that Obama has made to seek union involvement in pay-for-performance plans. The administration has proposed more funding in the next fiscal year for programs that experiment with merit pay.

Duncan and Weingarten made clear that teachers should be ready for such experiments.

"When we say, 'With us, not to us,' it means we have to be on the field," Weingarten said. "We have to be engaged."

The federation is the nation's second-largest teachers' union, with 1.4 million members, many of them in large cities such as New York and Washington.

In a speech July 2 in San Diego to the National Education Association, the largest U.S. teachers' union with 3.2 million members, Duncan challenged educators to be open to linking pay to performance and to experiments that could reduce job tenure protections. That address drew some boos. Yesterday, his audience responded more favorably.

To a Chicago teacher who wanted to know what would be done about social problems outside school that affect academic performance, Duncan said: "We all have to be part of the solution."

A New Jersey teacher asked why bilingual education gets little respect. "We have to remove that stigma," he said.

An Illinois teacher queried Duncan about standards for paraprofessionals. "Great question," he said. "One I haven't done a lot of work on."

When he talked about the successor to No Child Left Behind, Duncan appeared to take issue with some policies that put a public spotlight on schools that fall short of academic targets.

"This idea of labeling and stigmatizing schools as failures -- it is unbelievably demoralizing to faculty; it's confusing to parents," he said.

That line drew applause.