The dramatic rise in New York City’s fourth-grade reading scores – the highest one-year increase ever achieved by the city’s public-school pupils on the state English Language Arts exam – will no doubt serve as vindication of Mayor Bloomberg’s management of the public-school system. But it’s worth noting that the city’s charter schools led the testing gains, achieving even higher scores than public schools overall.

In the city at large, the percentage of fourth-graders meeting state reading standards rose 9.9 points to 59.5%. New York City’s public charter schools saw a rise of 13.2 points, for a total 60.9% proficiency. Eleven of the 16 charter schools with fourth-grade students outperformed their district averages, according to an analysis by the New York Center for Charter School Excellence.

On the eighth-grade reading test, the difference was even more pronounced. Citywide, the proportion of public-school pupils meeting state standards declined 2.8 points from 2004. Just 32.8% of city pupils passed the exam this year. Among those children enrolled in charter schools, however, 48.5% passed – a 5-point increase over last year.

Five of the six charter schools reporting eighth-grade scores outperformed their district averages, according to the New York Center for Charter School Excellence. The one charter school that did not outperform its district is a special program for under-credited students.

The Bloomberg administration has argued that many eighth graders did not benefit from the mayor’s education policies due to a poor education in the earlier grades. But the charter schools have evidently succeeded at educating even those middle-school pupils underserved by the public-school system at large.

The city’s Department of Education now plans to create a new $40 million “intervention strategy” to help struggling middle schoolers. But if the city is serious about improving student performance, it would be well-advised to establish more charter schools as an alternative to the traditional government-run system. Last year, Mayor Bloomberg and his schools chancellor, Joel Klein, pledged to open 50 new charter schools over the next five years. Currently, there are 32 public charter schools in New York City with 15 more approved to open in the fall.

But this educational success story faces a roadblock come December. That’s when the number of charter schools in New York State is expected to reach the statutory cap of 100 schools – a limit established by the 1998 law that first authorized charter schools in the state.

The cap can only harm New York’s children. Demand for charter schools is so great that virtually every school in the state has a waiting list, with almost 10,000 children waiting for a chance to enroll.

Assemblyman Vito Lopez, a Democrat of Brooklyn, has proposed legislation to lift the cap and allow school districts themselves to decide how many charter schools to approve. “Charter schools no longer should be viewed as an ‘experiment,’ but rather should be embraced as an effective component of the public education system,” reads the legislative memo accompanying his bill. “As such, charter schools should be encouraged to grow in New York, and the environment in which charter schools operate should be improved.” State Senator Martin Golden, a Republican of Brooklyn, has proposed similar legislation in the state Senate.

Last fall, the chairman of the Assembly’s Committee on Education, Steven Sanders, called lifting the cap “premature, given the fact that we haven’t reached the 100 yet.” Mr. Sanders, a Democrat of Manhattan, said he would be willing to remove the cap only when “all charter schools are up and running and we have experience with them.”

Meanwhile, New York’s Board of Regents has concluded that charter schools overall have improved student performance at a greater rate than their district counterparts, even while spending less per pupil and serving those at greatest risk of academic failure. To this evidence we can now add the results of New York’s English Language Arts exam this year.

Mr. Sanders, call your office. It’s time to lift the cap.