WASHINGTON — With the public education system in crisis, President Obama called Monday for purging underperforming teachers and lengthening the school year so that the United States keeps pace with other advanced countries.

He said more spending is needed to update textbooks, facilities and equipment, but added that money without reform would not solve the problems of education.

“”You can’t defend a status quo in which a third of our kids are dropping out,”” Obama said in an interview on NBC’s “”Today”” show. “”You can’t defend the status quo when you’ve got 2,000 schools across the county that are dropout factories — and they really are — where more than half of the kids are dropping out.””

Incompetent teachers must be identified and weeded out, he said in the interview, which took place in the White House.

“”We’ve got to be able to identify teachers who are doing well (and) teachers who are not doing well. We’ve got to give them the support and the training to do well,”” Obama said. “”And, ultimately, if some teachers aren’t doing a good job, they’ve got to go.””

Teachers unions remain an important part of the Democratic Party base. Yet, Obama’s view of unions, while positive, was also tempered.

“”I’m a strong supporter of the notion that a union can protect its members and help be part of the solution, as opposed to part of the problem,”” he said in the interview. “”What is also true is that sometimes that means they are resistant to change when things aren’t working.””

Unions in many states have been partners in finding solutions, Obama said, adding that sometimes “”radical change”” in schools is necessary.

Obama endorsed the ideas of merit pay for teachers and a longer school year in a major education address last year. Reiterating those views, Obama said in the interview that the extra cost of a longer school year would be worth it.

“”We now have our kids go to school about a month less than most other advanced countries,”” the president said. “”And that month makes a difference. It means that kids are losing a lot of what they learn during the school year during the summer.””

He added: “”It’s especially severe for poorer kids, who may not be seeing as many books in their house during the summers. So the idea of a longer school year, I think, makes sense.””

During the interview, Obama said he did not think his two daughters, who attend an elite private school in Washington, could receive as good an education in the local public schools.

In Washington’s recent mayoral primary, voters rejected first-term Mayor Adrian Fenty, who made a major effort to overhaul the district’s public schools, closing classroom buildings and removing teachers.

Obama said that, as president, he could probably find a public school able to provide his daughters an excellent education.

“”But the broader problem is for a mom or a dad who are working hard but don’t have a bunch of connections, don’t have a lot of choice in terms of where they live,”” he said. “”They should be getting the same quality education for their kids as anybody else. And we don’t have that yet.””

Obama also restated his support for the use of charter schools as an alternative to public schools, but added he wanted to make sure they were high quality and accountable.

“”Charter schools are not a panacea,”” he said. “”We shouldn’t say, just because a school’s a charter, that it’s an excellent school, because there are some actually very poor-performing charters.””