The National Assessment of Educational Progress results come amid the nation’s highest graduation rate, raising questions about whether a diploma is a meaningful measure of achievement. (Gabriella Demczuk/For The Washington Post)

The nation’s high school seniors have shown no improvement in reading achievement and their math performance has slipped since 2013, according to the results of a test administered by the federal government last year.

The results of the National Assessment of Educational Progress, or NAEP, also show a longer-term stagnation in 12th-grade performance in U.S. public and private schools: Scores on the 2015 reading test have dropped five points since 1992, the earliest year with comparable scores, and are unchanged in math during the past decade.

“These numbers are not going the way we want,” said William J. Bushaw, executive director of the National Assessment Governing Board, an independent panel established by Congress to oversee NAEP policy. “We have to redouble our efforts to prepare our students.”

The sobering news, released Wednesday, comes at the same time the nation is celebrating its highest-ever graduation rate, raising questions about whether a diploma is a meaningful measure of achievement.

Eighty-two percent of high school seniors graduated on time in 2014, but the 2015 test results suggest that just 37 percent of seniors are academically prepared for college course­work in math and reading — meaning many seniors would have to take remedial classes if going on to college.

[High school graduation rate hits all-time high]

“The governing board is pleased that graduation rates are increasing across the country, but at the same time not pleased that we’re not making the academic progress that we need to so there’s greater preparedness for post-secondary, for work, for military participation,” Bushaw said.

A spokeswoman for U.S. Education Secretary John B. King Jr. did not immediately comment.

Also known as the Nation’s Report Card, the NAEP is widely regarded as the most consistent measure of U.S. student achievement over time. Since the 1990s, it has been administered every two years to students in the fourth and eighth grades, and less frequently to high school students.

In 2015, average math performance among seniors slipped two points, to 152 on a 300-point scale. On the reading test, seniors posted an average score of 287 on a 500-point scale, which was not statistically different from 2013.

Results for fourth- and eighth-graders on the 2015 tests were announced in October. Like high school seniors, the younger students demonstrated lower performance in math compared with 2013. Reading performance dropped for eighth-graders and was flat for fourth-graders.

[U.S. student performance slips on national test]

The stagnation comes after a turbulent period in public education. Most states adopted sweeping educational policy ­changes, including teacher evaluations tied to test scores and Common Core academic standards that have changed what and how students learn in the classroom.

Last year, then-U.S. Education Secretary Arne Duncan defended those policies, saying that substantial reforms in schools often lead to a temporary drop in test scores while teachers and students adjust.

“Big change never happens overnight,” Duncan said. “I’m confident that over the next decade, if we stay committed to this change, we will see historic improvements.”

At all grade levels and in all subjects, there remain yawning racial achievement gaps. Those gaps did not narrow in 2015 among high school seniors, according to the test results.

In math, for example, 47 percent of Asian students and 32 percent of white students scored proficient or above, compared with 7 percent of black students and 12 percent of Hispanic students.

There was a widening of one achievement gap, between the highest performers, whose scores put them in the 90th percentile nationwide, and the lowest performers, at the 10th and 25th percentile.

In reading, the scores of top students improved in 2015 while the scores of low-performing students declined. Low-performing students posted a similar decline in math, while top students were unchanged.

“The students at the lower end are getting worse,” said Peggy Carr, acting commissioner of the National Center for Education Statistics, which is responsible for administering the NAEP. “That’s something we need to think about.”

Correction: An earlier version of this story reported that younger students demonstrated stagnant performance in math compared with 2013, when it should have reported that they demonstrated lower performance in math. The story has been updated.