D.C. Mayor Vincent C. Gray (D), seen here speaking last week at the South African embassy, attended his first event alongside fellow candidates for the Democratic nomination for mayor on Monday night — a forum on the city’s schools. (Cliff Owen/AP)

D.C. Mayor Vincent C. Gray (D) chose a debate on education Monday night to make his campaign debut alongside council members, a restaurant owner, a former State Department official and others trying to keep him from a second term.

He could have picked an easier place to start.

Before a packed auditorium at Eastern High School of teachers, union leaders and activists — many of them upset with Gray’s current schools chancellor, Kaya Henderson — Gray was welcomed with a question that immediately put him on the defensive.

“What is your criteria for selecting a chancellor?” asked President Elizabeth Davis of the Washington Teachers’ Union, which sponsored the forum.

“Our chancellor is a person who understands the importance of working with teachers,” Gray began, eliciting a smattering of boos and groans.

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“She was the first appointment that was made in my administration.” More groans.

As Gray got around to the heart of his response, the bell rang and he was cut off.

“When you look at the results, our test scores have gone up . . . .”

Gray’s voice trailed off. He set down the microphone and returned to his seat.

It was as good as it got for the rest of the night.

The hostile crowd showed that the city’s long-beleaguered school system — and tense negotiations with its teachers union — could complicate the incumbent mayor’s reelection narrative that the city’s schools have improved under his watch.

During Gray’s three years in office, the District’s public school system has recorded rising scores on standardized testing, and Gray has highlighted those achievements as evidence of his seriousness about education reform. He would continue on that path if reelected, he said.

But in holding up test scores as the barometer of success, Gray has exposed himself to the charge that he has abandoned his skepticism of reform from his 2010 campaign. Then, he said test scores were not the largest measure of success — helping to attract the strong backing of the Washington Teachers’ Union.

On Monday, upstart candidates including restaurateur Andy Shallal, the owner of Busboys and Poets, and Reta Jo Lewis, a Democrat and former State Department official, drew the biggest applause, indicting both Gray and members of the council who seek to replace him for alienating parents and teachers amid a forceful push for school reform.

Shallal criticized mayoral control of the schools, which was authorized in 2007, saying that it has led to untenably high teacher turnover and “changed the way we put the public in public schools — people have become more disenfranchised, disaffected and disrespected.”

He also took aim at the city’s support for school choice, in which many children face long odds to win admission by lottery to the most sought-after schools.

“I don’t want to play Russian roulette with our kids,” he said. “Every single kid deserves a good education.”

Lewis drew hoots of support when she declared that, since the D.C. Council granted then-Mayor Adrian M. Fenty power to take over the ailing school system, too many “secrets” have been kept about school management.

“We don’t know when you’re going to close the schools, we don’t know why you’re going to close the schools . . . and then when the planning process takes place they never come to the parents, they never come to the teachers, they never come to the community at large.”

“I want you to know that the pledge that I would make to this community . . . is that it’s going to be a city that works for everyone.”

Long-shot candidate Christian Carter even walked over to Gray and stared him down for dramatic effect and applause when asked to grade the city’s schools under the current administration.

“An F, ” Carter said.

Gray ran in 2010 as a critic of Fenty and Fenty’s handpicked chancellor, Michelle A. Rhee — and their aggressive effort to overhaul the city’s public schools, including dozens of school closures and new teacher evaluations that linked pay and job security to students’ standardized test scores.

“What this administration has done is create an environment in which teachers are teaching to the test, and I don’t think that is necessarily a measure of whether kids are learning or not,” Gray said in a debate with Fenty on WAMU-88.5’s Kojo Nnamdi Show four years ago.

Yet instead of dismantling Rhee’s reforms, Gray has continued them. His chancellor — Henderson, a Rhee deputy — has closed more than a dozen schools and has fired hundreds of teachers for poor performance.

Gray champions these policies — along with universal preschool, which he pushed through as council chairman, and the city’s growing charter-school sector — as successful.

Not everyone in the audience agreed with the union’s skepticism of standardized testing and increasing reliance on charter schools.

“It feels like they’re putting forth a vision like we’re all united, but it would be nice to see different points of view,” said Angel Ballard, a second-year chemistry and special education teacher at Eastern High.

Nonetheless, hundreds of teachers in the audience loudly booed Council member Jack Evans (D-Ward 2) when he offered his support for mayoral control, saying it had finally interrupted a cycle of finger-pointing that made the city’s school system one of the nation’s worst.

Evans asked for 20 more seconds because he said the audience was interrupting him. It only drew more boos.

Council member Tommy Wells (D-Ward 6) argued that everyone deserved a good elementary school within walking distance in the city.

“The question of mayoral control has a lot to do with who’s mayor,” said Wells, who added that the increasing popularity of elementary schools in his home ward is evidence that he has included parents in decision-making.

“I know how to turn DCPS schools around because we’ve done it,” he said.

But the questions kept coming around to Gray, who appeared to have few allies in the room.

“Ladies and gentlemen,” Gray began his last answer on mayor control, “some things are not popular.”

Gray rejected the claims that mayoral control has been a failure, pointing to recent national test gains as well as the strides the city has made in providing public preschool for 3- and 4-year-olds.

“We have the greatest growth in improvement in our children than any state in America,” he said. “Some things that you do may not be popular with everybody, but the thing that ought to be popular with everybody is we want positive outcomes for our children.”

Chuck Thies, Gray’s campaign manager, said the teachers union originally invited Gray to speak as mayor at the event. After he declared his bid for reelection last week, the union altered the format and asked him to speak instead as a candidate and extended invitations to more of his challengers.

The mayor had not intended to participate in any debates until after the new year, Thies said, but felt he could not say no after initially agreeing to attend as mayor. If Gray were invited to a similar event again, Thies said he would want to talk with organizers and ensure “a more controlled” environment. “If you were in the back, you couldn’t hear anything,” Thies said, referring to the outbursts by the crowd.

“But if this race is about education, we’re happy to go toe-to-toe on that topic all the way. . . . no one has the record of Mayor Gray.”

Council member Muriel Bowser (D-Ward 4) did not attend the forum because of a scheduling conflict.