Fourth- and eighth-graders in the United States have made little to no gains in math and reading since 2015.

While the average reading scores for eighth-graders increased compared with 2015, there were no changes for reading at fourth grade or for math at either grade, according to results from the 2017 National Assessment of Education Progress, also called NAEP or The Nation's Report Card.

Moreover, the latest results reveal a disturbing trend in which the country's poorest-performing students scored worse in both subjects than they did in 2015, while the highest-performing students posted increases, reflecting a growing gap between those at the top and bottom of the achievement spectrum.

"I'm pleased that eighth-grade reading scores improved slightly but remain disappointed that only about one-third of America's fourth- and eighth-grade students read at the NAEP Proficient level," said former Gov. John Engler of Michigan, the chair of the National Assessment Governing Board, which along with the National Center on Education Statistics, the Department of Education's research arm, administers the test.

"We are seeing troubling gaps between the highest- and lowest-performing students," he said. "We must do better for all children."

To be sure, results varied considerably among states and the 27 large urban school districts that volunteered to have their scores individually analyzed and included in the 2017 Trial Urban District Assessment, which was also released Tuesday and is known as TUDA.

When it comes to breaking down NAEP scores by state, this year Florida was the stand-out.

Florida was the only state to see an increase in math, as the average scores of both fourth- and eighth-graders increased between 2015 and 2017. Most states' average scores remained unchanged in math, though 10 states saw declines in fourth-grade math and three saw declines in eighth-grade math.

Most states' average scores were also unchanged in reading, with the exception of 10 states whose eighth-graders posted increases.

"Something very good obviously is happening in Florida," said Peggy Carr, the associate commissioner of assessment at NCES, though she stopped short of correlating those increases with any specific policy change. "Florida needs to be commended."

In addition to an uptick in the Sunshine State's math and reading scores, Florida saw increases in almost all student subgroups inching up their proficiency rates, including students of color, low-income students, students with disabilities and those still learning English.

Moreover, two of Florida's three large school districts that volunteered to participate in TUDA, Duval County and Miami-Dade, also posted similar increases.

"I can't tell you why," Carr said. "But something interesting is happening in Florida."

Tuesday's results, however, were undercut by the fact that the test was administered digitally for the first time via a tablet device. Research shows students tend to score worse on digital assessments than on traditional paper tests, prompting some state education officials and policymakers to dismiss the results ahead of their release, despite Carr's insistence that NCES researchers properly accounted for the change.

"We're going to learn a lot more about what students know and can do, not just their answers, but more about how they arrived at these answer through this more digitally based assessments," Carr said to reporters on a press call Monday.

When researchers at NCES analyzed the scores more than 200 times and compared them to the smaller cohort of students who took the test as it has been traditionally administered with paper and pencil, they found very few inconsistencies with the results. In fact, Carr said, in the handful of the inconsistencies they did find, it was often the case that students who took the test digitally performed better.

"We are just ecstatic about being able to move these assessments to a digitally based format," she said. "Students are communicating, living, they learn and are taught in a digitally-based world, so assessments such as NAEP are moving toward a digitally based assessments."

Ahead of Tuesday's results, policymakers and advocates were bracing for students to fare worse than in years' past, concerned about what the the country's most cited indicator of student achievement will mean for the trajectory of education policy across the country, despite a chorus of attempts from education researchers to caution against making causal claims.

"We've come to anticipate NAEP results as an indicator of student academic achievement, but we shouldn't base our perceptions of education in America, or in individual states or cities, so heavily on this one data point," said Chris Minnich, the CEO of NWEA, an organization that designs K-12 assessments.

"The concerns I hear from education leaders center on making sure we use multiple measures of student learning to inform our opinions on how our schools, districts, and states are doing," he said, stressing that student growth data is a better representation of education progress.

In addition to reporting math and reading scores by state, this year's release also includes the results of fourth- and eighth-graders in 27 urban school districts that volunteered to have their scores reported out separately via the 2017 TUDA.

Again, mirroring the flat-line trend that occurred among states, most of the average scores for the city school systems remained unchanged in both subjects since the last assessment.

A handful of outliers include San Diego, where fourth-graders increased their average scores in math and reading; Duval County, Florida, Fresno, and Miami-Dade, where fourth-graders posted increases in math; Charlotte, Cleveland, Dallas and Detroit, where average scores declined in fourth-grade math; and Albuquerque and Boston, where eighth-graders increased average reading scores.

"Today's release of The Nation's Report Card confirms that there is still much work to be done to close achievement gaps and ensure that our young people are ready for success in college, careers and life," said Carissa Moffat Miller, executive director of the Council of Chief State School Officers, the organization that represents every state's top education official.

"State chiefs recognize the urgency of improving outcomes for all students, and these recent results from the Nation's Report Card only further demonstrate this call to action," she said.