Staples High School is failing to meet requirements under the federal No Child Left Behind law because math test scores by a "subgroup" of sophomores last year failed to show "adequate yearly progress," according to a report released Monday.

Among the group of 50 special education sophomores, 12 did not meet the math proficiency standard on the Connecticut Academic Performance Test, resulting in a 76 percent proficiency rate. This year, the NCLB law set a group benchmark of 91 percent proficiency in math.

That was the lone group of Staples students who did not test at or above the NCLB standards.

"It can happen from year to year. We're going to investigate and study why this happened," said Westport Superintendent of Schools Elliott Landon. "We'll try to do better next time."

In addition to the math standard, 89 percent of students must show proficiency on the CAPT's reading component. The targets must be met by the entire school, as well as all subgroups with more than 40 students, including special education students, English language learners, economically disadvantaged students, and black, white, Asian and Hispanic students. The test scores are split into five performance levels: below basic, basic, proficient, goal and advanced.

The rest of Westport's public schools achieved the increasingly demanding standards set by the law, although a growing number of schools across the state fell short.

Over a year's time, the number of high schools statewide not meeting adequate yearly progress grew from 61 to 86 among 193 statewide, and the number of elementary and middle schools failing to meet the standards bumped up to 373 from 220 out of 786 statewide.

"The federal law is a strict, across-the-board, absolute bar of 100 percent proficiency by 2014, so there are increasing demands," Mark Linabury, a spokesman for the state Department of Education, said Monday.

The goal of the decade-old law approved with bipartisan support during the Bush administration is to have all students proficient in math and reading in 12 years.

In Connecticut, every couple of years, the percentage of students who need to be proficient to avoid the label of "not making adequate yearly progress" grows.

Last year in reading, schools were safe if 79 percent of students achieved proficiency on the Connecticut Mastery Test given in grades three through eight and on the Connecticut Academic Performance Test given in grade 10. This year, that percentage is 89 percent. In math, the bar has risen from 82 to 91 percent.

This is the first year Staples failed to make the "average yearly progress" benchmark. But Westport is not the only district that had trouble meeting the increasingly stringent federal standards. Nearby Darien and New Canaan were "all clear" this year, but elsewhere in lower Fairfield County, districts did not fare as well.

Stamford was one of 34 districts that failed to make AYP due to the whole district's math and reading scores; Greenwich schools also were among the 10 districts across the states to miss AYP due to subgroups' performance on this year's round of standardized testing.