A new study of test scores found that, between the third and eighth grades, Seattle students make nearly six years of academic growth. In less-afluent Highline schools, the student growth rate is even higher.

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Two years ago, a Stanford study of student test scores from the nation’s 200 largest public-school districts showed that Seattle had the fifth-widest gap in achievement between black and white students.

On average, the researchers found, black students in Seattle Public Schools score three-and-a-half grades behind their white peers on standardized tests. And after updating that 2016 report with more recent test scores, the Stanford team found Seattle’s black-white gap has widened to 3.7 grade levels.

But it’s not all bad news for Seattle. In December, the same researchers released another study that showed, overall, students in Seattle make nearly six years’ worth of growth on math and reading tests in five school years.

That’s the third-highest growth rate among the 200 large districts, according to The New York Times, which first reported the study’s findings last month.

In its new study, the Stanford Center for Education Policy Analysis looked at student test scores from 2009 and 2015.

The study found that third-grade students in Seattle score 0.6 years above the national average. By grade eight, they score 1.3 years above — reflecting 5.7 years of academic growth over the five years of school.

Only Chicago Public Schools and the Chandler school district in Arizona posted higher growth rates, at six years and 5.8 years, respectively.

Researchers also looked at all school districts nationally and found that, in Washington state, several other districts also had higher growth rates than Seattle’s. Seattle actually ranked sixth among the largest 20 districts in Washington, behind Bellevue, Everett, Kent, Pasco and Highline.

In Highline, for example, students start third grade 0.8 years below the national average and yet, five years later, test on par with their peers across the nation, reflecting 5.8 years of growth. That’s notable given that the median household income is about $50,600, a third less than Seattle.

“Where students start in third grade is very related to their socioeconomic status,” said Sean Reardon, a professor of poverty and inequality in education at Stanford, noting that affordable child care, high-quality preschool and the home environment can influence how far ahead or behind a student is by elementary school.

“The growth stuff is probably more related to the school district,” he added. “It suggests that there may be pretty effective school districts in lower-income communities and some (not-so-effective) districts in more affluent communities.”

The latest Stanford study also compared growth rates among racial groups of students.

In Seattle, the researchers found, black and white students make the same amount of growth between third and eighth grade. Asian and Latino students in the city, meanwhile, slightly outpace their black and white peers. But that might just mean that, as Seattle grows more expensive, poorer families move out of the city and, by eighth grade, the results reflect a greater percentage of students from higher-income families.

The new growth data also suggested to Reardon that parents and policymakers shouldn’t rely on a single year of test scores to judge the quality of a school or district.

“If you just look at the average test scores or proficiency rate in a district, it might not be a very good signal of the quality of the schools themselves,” he said. “It might be more a reflection of all the other kinds of opportunities that kids have or don’t have at home.”