Public schools in New Orleans are significantly better since Hurricane Katrina but gains began to level off in 2013, according to a report issued Tuesday morning.

English scores have been stagnant in English and dropped in math in the past six years, the Education Research Alliance for New Orleans concluded.

The organization was set up to analyze post-Katrina school changes after the 2005 hurricane leveled the long-troubled New Orleans public school system.

The 14th anniversary of the storm is Aug. 29.

In 2006 the state took over most of the city's public schools and 98 percent of students now attend charter schools, which are public schools are that are supposed to offer innovative teaching methods.

The report says school gains have come through internal changes and by closing under-performing schools and moving students to new sites.

"Our analysis suggests there is still room for New Orleans schools to improve through both of these mechanisms," according to the study.

"The performance of new schools continues to be higher than schools that have closed or been taken over," it says.

"However, the impact of such extreme measures will very likely be smaller than in the past. Closure and takeover alone cannot produce a high-quality education system."

Why elementary and middle school performance in English plateaued and dropped in math is unclear.

Some educations attributed the decline to a change in standardized tests and problems keeping quality teachers, the report says.

About 49,000 students attend public schools in New Orleans, down from about 65,000 before the hurricane.

The group that did the report is based at Tulane University and was founded by Douglas N. Harris, who is also one of the authors of the review.

Advisers to the organization include a wide range of public school groups, including the Louisiana Association of Public Charter Schools, Louisiana Association of Educators and the Louisiana Federation of Teachers.

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