National test scores declined for the lowest-performing high school seniors in math and reading, and more than 6 in 10 seniors were not academically prepared for college, according to new data released Wednesday.

Higher-performing students held steady or improved from past years, a pattern visible in other tests administered by the National Assessment of Educational Progress, or NAEP.

One likely explanation for the results is a significant decrease in students dropping out of high school, meaning the cohort of students tested included more lower-achieving students. In earlier years, these students might have left school and been excluded from the testing, experts said.

But this does not fully explain the results, they said, and does not explain declines in younger grades reported previously.

"In both math and reading, we notice a pattern of declining scores concentrated among lower-performing students," said Peggy Carr, associate commissioner of assessment at the National Center for Education Statistics. "This is a troubling indication that too many students have fallen behind and it's something we want to know more about."

The NAEP, sometimes referred to as the "nation's report card," is a closely watched exam because it assesses the performance of children from all racial and socioeconomic backgrounds in urban, suburban and rural communities. The government first administered a version of the exam in 1990.

Exams were administered in 2019 to 52,100 12th-graders at 3,550 schools, with about half taking the math test and about half the reading test.

For reading, the test results suggest 37% of high school seniors are academically prepared for college, meaning ready for college-level work without remedial courses. The portion prepared in math was also 37%.

Reading scores fell between 2015, the last time the test was administered to seniors, and 2019. That was driven by declines among the lowest-performing boys. For instance, boys in the 10th percentile saw a statistically significant decline of six points on the 500-point exam.

Higher-performing students saw no change in their scores since 2015.

The average math score was unchanged, but again, students in the bottom percentiles recorded declines.

The same pattern was apparent in comparing 2019 scores with results from earlier years: The lower-achieving groups declined while higher-achieving groups remained steady or improved.

Carr said that students in the bottom percentiles tend to be from lower-income families and were disproportionately Black or Hispanic.

Overall, the tests show fewer than half of all students rated below proficiency, which is defined as demonstrating competency over challenging subject matter.

For reading, 37% of students ranked proficient or advanced, with 24% at least proficient in math.

In both cases, there were increases in the number rated below the "basic" score, the lowest of four buckets. In reading, 30% were rated below the basic level, and in math, 40% were below basic.

The test also asked students about the reading they do outside of school. It found about 1 in 4 high school seniors never read stories or novels and about half never read poems on their own. About 1 in 4 read stories or novels at least once or twice a week, with 14% saying the same about poetry.