In reading, almost 4 in 10 students reached the "proficient" level or higher.
In both subjects on the 2013 exam there was little change from 2009, when the National Assessment of Educational
Progress was last given to 12th-graders. The results, released Wednesday, come from a representative sample of 92,000 public and private school students.
The stagnation is "unacceptable," said David Driscoll, chairman of the National Assessment Governing Board, which sets policy for the exam.
"Achievement at this very critical point in a student's life must be improved to ensure success after high school," Driscoll said.
The results follow the just-released and seemingly more encouraging research that U.S. high school graduation rates in 2012 reached 80 percent, a record.
John Easton, acting commissioner of the Education's Department's National Center for Education Statistics, said one possible reason was that lower-performing students who in the past would have dropped out remained in the sampling of students who took the exam.
In reading, the 38 percent share of students performing at or above proficient was lower than when the assessment was first given in 1992, when it was 40 percent. Scores have remained similar since 1994.
Past comparisons in math date only to 2005. Scores increased from 2005 to 2009.
The student participants' response to a survey about their educational experiences offers some clues to their performance.
Among the findings:—Students who reported rarely or never discussing reading interpretations in class had average scores lower than those who did daily or almost daily.
—An overwhelming majority reported that reading is enjoyable. Students who strongly disagreed with the idea that reading is enjoyable had an average score much lower than those who strongly agreed.
—Math scores were higher, on average, for students who took calculus and lowest for students who had not taken a math course beyond Algebra I.
—Math scores were higher for students who reported math was their favorite subject, believed the subject would help them in the future or thought their class was engaging.
Even as 12th-grade scores have stagnated, fourth- and eighth-grade students have made slow but steady progress on the exam since the early 1990s; most progress has come in math.
Michael Petrilli, executive vice president at the conservative-leaning Thomas B. Fordham Institute, said it's unclear why younger students are doing better, but not high school seniors.
"This is one of the great mysteries of education today is why are we not seeing the same improvements at the 12th grade level as the fourth- and eighth-grade level," Petrilli said.
One speculation is that high school seniors aren't motivated when they take the exam. Another is that students are taking watered-down classes and "all we've done is put them in courses with bigger titles," said Mark Schneider, the vice president at the American Institutes for Research who is the former commissioner of the National Center for Education Statistics.
At all levels, there continue to be racial disparities. Petrilli said there needs to be a closer look at how demographics have changed and how each group is faring.
Among high school seniors, white and Asian students scored higher on average on the recent results in both reading and math than black, Hispanic and American Indian students. Asian students scored higher than white students in math but did not do significantly better in reading. As in past years, male students did better than female students in math, but females outperformed males in reading.
These results did not include a global comparison, but U.S. students historically do poorly on international assessments compared to many global peers.
Education Secretary Arne Duncan said in a statement that even though there has been some good news related to graduation rates and scores in younger grades, high school achievement has been flat in recent years.
"We must reject educational stagnation in our high schools, and as a nation we must do better for all students, especially for African-American and Latino students," Duncan said.
The results come as community colleges and four-year institutions try to improve remedial education programs, given that only about one-quarter of students who take a remedial class graduate.
It's estimated that more than one-third of all college students, and more than one-half in community colleges, need some remedial help, according to research from the Community College Research Center at Teachers College, Columbia University.
In additional to the national scores, 13 states voluntarily participated at a greater level and had scores reported.