City students' scores on national reading and math exams have flatlined since 2009, officials announced Wednesday. The surprising news delivered a setback to the Bloomberg Administration, which has consistently trumpeted the positive results of its school reform agenda.

The National Assessment for Educational Progress is a set of exams administered every two years to the nation's 4th and 8th graders and is considered the gold standard for measuring academic achievement. The 2011 results had city officials scrambling to explain the results.

Department officials noted that there has been significant progress among 4th graders since Mayor Bloomberg implemented reforms in 2003, and they emphasized that city students kept pace with or improved compared with the rest of the state over the past two years.

Still, 8th-grade reading scores have showed no significant improvement since 2003, calling into question whether the gains made by 4th graders disappear by the time students reach middle school. This year only 24 percent of 8th graders reached the level of proficient or better in reading, compared with 22 percent in 2003. And 24 percent were proficient in math compared with 20 percent in 2003.

"It's a challenge to all of us to re-double our efforts," said Shael Polakow-Suransky, the Chief Academic Officer for the city's Department of Education.

Although they made no gains over the past two years, city 4th graders have fared better overall, in keeping with the national trend. This year 29 percent of city fourth-graders reached proficient in reading compared with 19 percent in 2002, and the percent proficient in reading climbed to 32 percent from 21 percent in 2003.

Nationally, urban students have seen some improvement since 2009 in math while reading scores have stayed flat.

Some education experts said that changes in curriculum and district supervision made in 2007, and the lack of consistent incremental reform, have had negative results.

"I am very concerned," said former top Education Department official Kathleen Cashin, now a member of the state Board of Regents. "We have to come up with collaborative instructional strategies and support for our teachers. I think, as Debbie Meiers says, the approach needs to be data-informed not data-driven."

Officials said that initiatives currently in place, such as curriculum changes and plans to open 50 new middle schools, would help boost future scores.

The racial achievement gap between white and black or Hispanic students also has not budged significantly since 2003, but city officials noted that low-income 8th-grade students have made some strides closing the gap with higher-income students.