Principals need more support to be effective school leaders, agreed five candidates running for mayor at a forum Wednesday night.

And part of that support, the candidates said, included training principals on how to identify effective teaching and mentoring teachers to improve their craft.

The forum was sponsored by the Council of School Supervisors and Administrators, the union that represents principals. Participants included Tom Allon, president of the community newspaper group Manhattan Media; Bill de Blasio, the public advocate; John Liu, city comptroller; Christine Quinn, speaker of the City Council; and Bill Thompson, former head of the Board of Education and former city comptroller.

(John Catsimatidis, who heads Red Apple Group, was expected to attend but did not.)

The candidates — not all of whom have officially declared their run for mayor — outlined their views at a similar discussion in November.

The forum came during a time of heightened tension between Mayor Michael Bloomberg and teachers’ and principals’ unions, after talks broke down over a new evaluation system earlier this month. The discussion also came just ahead of the city’s next round of school closure and co-location hearings which, if the past is any indication, will draw the ire of school communities under threat.

Liu was one of the candidates who said the next mayor must work more collaboratively with educators and parents.

“It just was too strange that the chancellor and the mayor often would just get up and announce these school closures as if they had achieved something for the educational system,” said Liu, who has called for a moratorium on school closures along with Thompson. De Blasio also called school closures under the Bloomberg administration “a bankrupt policy,” with no input from parents.

The moderators, Prof. Pedro Noguera of New York University and Liz Willen of The Hechinger Institute, asked the candidates how they would work with the unions to come up with a fair system to evaluate teachers and principals. The respondents took the chance to vent their frustrations over the breakdown in talks and subsequent loss of state education funding.

“We don’t let students fail, we call out schools excessively when they fail,” said Quinn. “We shouldn’t be allowed to fail. It’s the exact wrong type of leadership model.”

Allon said the mayor of New York City must get along with the teachers’ union as a fundamental starting point.

“It’s like a marriage,” said Allon. “If they don’t make it work, the kids will suffer.”

The candidates also touched on these topics:

—Co-locations: They have become too polarizing and contentious. Principals sharing space and parent associations must work collaboratively, said Allon.

—Mentoring: Quinn wants expert, veteran teachers to be able to spend two years as a “master teacher” with new educators.

—Pre-K: De Blasio plans to give all children access to full-day pre-K and better fund after school programs. He wants to tax the very wealthy to pay for it.

—The next chancellor: All candidates but Quinn affirmed the chancellor must be an educator.

—Vocational training: Allon drew widespread applause when he voiced support for expanding vocational education.

—Merit pay: Allon, to the chagrin of the audience, reiterated his support for merit pay for teachers and principals.

—Testing: Schools are too focused on high-stakes exams. All candidates agreed, but did not express how they would change this.

—Common Core: The only allusion to the new learning standards was from Liu, who said his seventh grade son is learning the higher order thinking skills to succeed in life. “How else could a 12-year-old muster such incredible sarcasm?”

—The Bloomberg legacy: On the lightening-round question of whether or not schools have improved under Mayor Bloomberg, Quinn and Allon said yes, compared to where schools were before. Thompson answered no, given the amount of progress that he said “should have been made under mayoral control.” De Blasio said yes during the first term, but then the schools have slid backward in the last several years. Liu said the numbers do not show that there has been success overall.