Math scores in schools nationwide have dropped for the first time in 25 years, prompting federal officials to speculate that results stemmed in part from a recent changeover to more challenging Common Core academic standards.

The National Assessment of Educational Progress, a semi-autonomous testing arm of the government, reported Tuesday that average math scores dipped both in the fourth and eighth grades between 2013 and 2015. This was the first recorded decline since the agency began collecting student sampling data in 1990.

Math scores also were down in New York and dozens of other states, at one grade level or the other, as were eighth-grade English scores.

U.S. testing officials, while acknowledging the math decline was unexpected, noted that 2015 scores remained well above 1990 levels. They also cautioned that a single recorded drop over a two-year span did not necessarily signal a trend.

Still, Education Secretary Arne Duncan and others within President Barack Obama's administration, who have pushed states toward tougher curriculum and testing standards for several years, voiced concern that critics might seize upon the results as evidence of failure.

"I wouldn't be surprised if there are folks out there who will say 'Turn back the clock,' " Duncan said during a phone conference with reporters. "That would be a big mistake."

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Criticism came swiftly.

Diane Ravitch, a New York University professor who has helped spark an anti-testing movement on Long Island and elsewhere, said the scores came as no surprise.

"This is the result of the chaos inflicted on public education over the past 14 years, first by the Bush administration and now the Obama administration," Ravitch said. "Learning requires consistency and civility, not disruption."

Growing resistance among teachers, parents and students to standardized testing has taken a toll on leaders of the national educational reform movement that has combined more rigorous exams with stricter job evaluations of teachers and principals.

Duncan recently announced he would step down in December. On Monday, Merryl Tisch, chancellor of New York State's Board of Regents, confirmed widespread speculation in saying she will not seek another term on the policymaking board when her term expires at the end of March.

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Results from NAEP testing, which draws on samples of hundreds of thousands of students and is scored on a scale of 0 to 500, show:

Fourth-grade math scores nationally dropped to an average 240, from 242 in 2013. New York's average dipped to 237, from 240.

Eighth-grade math results nationally went down to an average 282, from 285 in 2013. New York's average declined to 280, from 282.

Fourth-grade reading scores nationally rose to an average 223, from 222. New York's average dropped to 223, from 224.

Eighth-grade reading results nationally declined to an average 265, from 268. New York's average was down to 263, from 266.

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U.S. officials took a cautious approach in trying to explain the drops, not even mentioning the Common Core standards by name. The standards were developed under the direction of the National Governors Association, not the federal government, but received a big push from the Obama administration.

William Bushaw, executive director of the National Assessment Governing Board, which oversees federal testing, speculated that rapid curriculum changes might have contributed to a temporary drop in scores.

Those changes occurred as most states wrote and sometimes rewrote new classroom lesson guides to reflect the new national standards.

One complication is that the National Assessment's tests are based on that agency's own academic framework, not on the newer Common Core standards. Thus, the two systems are not perfectly aligned, though they cover much of the same academic material.

New York and some other states, such as Massachusetts, have tied their tests closely to the National Assessment's system by setting their cutoff scores for measuring student proficiency close to the assessment's levels.