HOUSTON — Ten Democratic presidential hopefuls talked about their plans to improve public education Friday at a union forum, promising to spend billions of dollars in new investment to raise teachers’ salaries, expand early-childhood education and relieve student debt and campus buildings.

Former vice president Joe Biden and Sen. Kamala D. Harris (Calif.) both participated, but they did not continue a contentious discussion about school busing that started this past week in the first debate among Democratic presidential candidates.

In fact, neither raised it — nor the other’s position — at the forum, which took place during the annual convention in Houston of the National Education Association, the country’s largest labor union, with more than 3 million members.

The only candidate to directly address busing was former Housing and Urban Development secretary Julián Castro, who said “voluntary busing” would be one way to help integrate America’s segregated schools.

“Today, we are still grappling with some of the same issues we were grappling with 40, 50 years ago,” said Castro, noting that fixing public schools would require attention to housing and other policies. “We don’t exist in silos, and neither should our policies.”

The other candidates at the forum were Sen. Bernie Sanders (I-Vt.), Washington Gov. Jay Inslee, Sen. Amy Klobuchar (Minn.), former Texas congressman Beto O’Rourke, Sen. Elizabeth Warren (Mass.), New York Mayor Bill de Blasio and Rep. Tim Ryan (Ohio).

None of the 10 candidates said anything negative about their rivals, but instead they used questions as opportunities to talk about their plans to boost public education, which all of them declared a priority. They each had 10 minutes, and all were frequently interrupted by applause from the thousands of union members in the audience.

Several candidates, including Biden and de Blasio, joined an earlier call from Warren to hire an educator as education secretary.

Harris, when asked what questions she would pose to candidates for the job, joked, “I will not be interested in grizzly bears.” That was a reference to a now-famous remark by Education Secretary Betsy DeVos. When asked in 2017 by a lawmaker about guns in schools, she said they might be needed to protect against grizzly bears.

DeVos was cited by a number of the candidates when drawing distinctions between her emphasis on expanding alternatives to traditional public schools and their stated intention to shower them with resources.

NEA President Lily Eskelsen García, who greeted each candidate and asked the questions, said in an interview after the forum that other candidates would have a chance to speak to union members or leadership and that the union was in touch with each of them.

Eskelsen García said in the interview that she was heartened that many of the candidates told her union they would seek input from educators when making policy.

“We’re in the classrooms. We hear our kids who can’t go to the doctor. We hear kids who have to stay home to take care of their younger kids because mom has to go to work and there’s no money to pay for day care,” she said. “Nobody ever asks us. What these folks are looking for is someone who will respect their professionalism and their expertise.”

She added: “To have these candidates say, ‘I respect your judgment before I would make a major decision on what I want to do with schools, I’ll ask you what your opinions are,’ that means everything to us.”

She said the union will take its time making an endorsement. “We want to see which campaigns are going to make it to the finish line,” she said.

Both the NEA and the American Federation of Teachers, the second-largest teachers union in the country, endorsed Hillary Clinton early in the 2016 campaign, angering some members who were Sanders supporters. Both unions are now holding forums for candidates to give them an opportunity to present their cases.

Each candidate pledged to spend varying billions of dollars for different objectives. They said they would raise the salaries of teachers, who they all agreed were sorely underpaid. They also pledged to provide more resources for schools in low-income neighborhoods, expand early-childhood education and eliminate crushing student debt.

A number said they would pay for their programs by rolling back President Trump’s tax cuts for the wealthy.

There were some differences in responses: For example, Sanders pledged to eliminate all student debt, while Harris talked about targeting middle- and lower-income debt holders. Sanders called for an end to public funding of for-profit charter schools, while de Blasio said no charters should get federal funding. But all candidates said education was a key issue.

With deference to the union crowd, some candidates spoke about the need to empower unions and to help workers in every state have the right to collectively bargain. Ryan said he would work to “double” the size of unions. None of them, however, detailed what actions they would take to empower unions.