In December 2003, parents in Beverly, Massachusetts, got a rude shock. A small, affluent city about half an hour north of Boston, Beverly was widely regarded as having one of the strongest urban school districts in the region. But a state Department of Education report that month told a different story. In the jargon of No Child Left Behind, the newly passed federal education law, Beverly students were failing to make “adequate yearly progress” on standardized tests. In simpler language, Beverly schools were failing.

The problem wasn’t Beverly’s test scores overall, which remained comfortably ahead of statewide averages. Rather, it was that not all groups of students were experiencing the same success. Just 17 percent of low-income 10th-graders, for example, were considered “advanced” or “proficient” in English, compared with 65 percent of students overall. Black, Hispanic and special-needs students also fell short of federally mandated improvement goals in one or more subjects.

In previous years, those groups’ struggles had been masked by the stronger results of the overall student body. But No Child Left Behind mandated that districts break out scores for individual “subgroups” — racial and ethnic minorities, low-income students and those in special education programs — and required each one to make gains.

“We thought we were doing well because the district overall was meeting all of the performance goals,” district Superintendent William H. Lupini told The Boston Globe at the time, “but then we got this subgroup data and we said maybe we need to take a closer look at what we’re doing.”

No Child Left Behind, which President George W. Bush signed into law 14 years ago next month, took aim at what Bush called “the soft bigotry of low expectations” by seeking to raise educational standards across the board. The law required that all U.S. students be proficient in reading and math by 2014, and put in place a system of penalties for schools and districts that didn’t make progress toward that goal. For the first time, the law judged schools based partly on the success of their weakest-performing students.

Nearly a decade and a half later, No Child Left Behind is often described as a failure, and there is no question that the law fell short of many of its most ambitious goals. Most schools didn’t come close to achieving the 100-percent-proficiency mandate, which experts never considered a realistic target. Subsequent research found that the law’s penalties did little to improve student performance, and may have done more harm than good in some schools. Large achievement gaps remain, in part because Congress didn’t provide all of the billions of dollars in additional education funding that the law’s backers envisioned.

This month, Congress closed the book on No Child Left Behind for good when it passed the Every Student Succeeds Act, which stripped away many of the old law’s most rigid requirements, including dismantling the metric that labeled Beverly’s schools a failure more than a decade earlier. Even before the new law passed, the education debate had largely moved on to fights over the Common Core curriculum and the role of testing in teacher evaluation.

But for all its failures, No Child Left Behind had at least one significant — and, experts say, lasting — success: It changed the way the American educational system collects and uses data. The law may not have achieved the promise of its title, but it did force schools across the country to figure out which students were being left behind, and to make that information public. Education experts argue that the law’s true legacy is the way it laid bare the inequities in the American educational system, and forced districts, in some cases for the first time, to address them.

“There’s a very long history of states and school districts and schools essentially hiding behind the average performance of their students,” said Scott Sargrad, a former Education Department official in the Obama administration who is now a researcher at the left-leaning Center for American Progress. “That masks really significant differences between kids who are more affluent, who are white, who don’t have disabilities, whatever it is, and their peers who are more disadvantaged.”

In some districts, merely drawing attention to racial or other disparities was enough to drive real changes. No longer able to coast on the strength of their high-performing majority, districts such as Beverly adopted new programs aimed at identifying and helping struggling students who might otherwise have fallen through the cracks. By 2005, Beverly was meeting the law’s standards for progress for all racial and income groups, and the performance gap among the groups had narrowed.

Beverly had some important advantages. It is a relatively wealthy district in a state that spends more per student than all but a handful of others. That meant once it had the data to identify struggling students, Beverly had the resources needed to help them.

“Once these affluent, well-performing school districts get the message that there’s something wrong with their performance, they have the capacity, they can move resources around enough that they can get themselves out of trouble,” said Thomas Ahn, a University of Kentucky economist.

Other districts weren’t so fortunate. In a paper published last year, Ahn and fellow economist Jacob Vigdor found that for districts without sufficient resources, the law’s sanctions — which included allowing students to transfer out of failing schools — merely made it harder to improve. Vigdor likened the law’s system of ever-increasing punishments to yelling at a failing student.

“You might succeed in scaring the Dickens out of the kid, but you’re not going to help them pass algebra,” Vigdor said, adding that the law didn’t do enough to address the racial and socioeconomic disparities that exist long before students enter first grade.

The law’s approach to data was also, by necessity, unsophisticated. When No Child Left Behind was signed in 2002, most states didn’t have the ability to track individual students from one year to the next. So instead, the law required schools to compare test results for whole classes across years — how this year’s eighth graders compare to last year’s, for example, even though each year’s students have their own particular strengths and weaknesses.

Today, states have improved their capacity to track students from year to year — the result, education experts said, of investments states made in part because of No Child Left Behind’s emphasis on data. The improved data makes it possible to see how students’ performance changes over time. That has given rise to new “value-added” models for evaluating schools and teachers, a controversial approach but one that has gotten high marks from researchers.

Although education policy has moved on from No Child Left Behind, its impact on data remains. Even as Congress abandoned many of the law’s provisions, it kept in place the requirement that districts report test results for individual groups. It will be up to interest groups and parents to use that data to push for change, said Lorraine McDonnell, a University of California, Santa Barbara, political scientist who recently published an article on the history of federal testing requirements. She said she expects the results to vary significantly from state to state and even school to school. But, she said, the American education system will never go back to talking about averages without paying attention to how vulnerable students perform.

“That’s been a pretty powerful legacy,” McDonnell said.

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