While the mayor last week touted the great strides made by the city’s public school fourth-graders on the state math test, it turns out charter school students fared just as well. Meanwhile, eighth-graders in charter schools did even better, with 66% passing the exam, compared with 40.8% of public school students citywide.

Nearly 77% of fourth-graders in both charter and public schools met or exceeded grade-level standards on the state math test, improving their average scores versus last year by about 9%.

Among eighth-graders, the proportion of charter school students meeting grade level swelled by 6.6%, while the number citywide dipped almost 2%.

The schools chancellor, Joel Klein, an ardent supporter of charter-schools, yesterday heralded the scores as evidence of “why we’ve worked so hard to make New York City the most charter friendly district in the country.”

In a written statement, Mr. Klein said, “It’s clear that our charters are continuing to deliver the impressive results that make them such stellar options for families.”

About 12,000 students attend the city’s 47 charter schools. Of those, nearly 90% are black and Latino and 75% are eligible for the federal government’s free lunch program. This year, 15 more charter schools opened across the city.

Run as independent public schools with their own boards of directors, charter schools are given a five-year charter that can be revoked by the state if academic goals are not met. They are exempt from following the citywide curriculum.

The rise in math test scores builds on the state scores released in May, which showed charter-school students making larger strides in reading than their peers at regular public schools.

Education experts cautioned against reading too much into the math score data released yesterday by the New York City Center for Charter School Excellence. The Department of Education released statewide scores last week.

“These tests don’t tell us anything,” an education expert and Manhattan Institute fellow, Sol Stern, said. He said it was impossible to evaluate a school’s progress based on one year of data. The averages were based on results from just six charter schools with eighth grade classes.

A leading scholar on school reform and a visiting scholar at New York University, Deborah Meier, said, “Test scores are only a small measure.” A supporter of charter schools, she said they needed to be evaluated on teacher and student retention and an examination of what happens to students after they graduate.

Charter-school leaders, however, were celebrating the test score jumps.

One of the middle schools that performed well was the Harbor Sciences and Arts Charter School in East Harlem, at which almost half of the eighth-grade class passed the state math exam.

The principal, Joanne Hunt, said her students were able to make great strides by having teachers take a step back. “We had to catch the students up,” she said. “They didn’t have the skills, but once we taught the basics, it was easy to catch them up.”

The New York State law on charter schools caps the number of schools allowed statewide at 100. Advocates used the data released yesterday to urge that the state remove that cap.