NASHVILLE (AP) - Tennessee Education Commissioner Kevin Huffman said Wednesday that a report showing the state's high school seniors' below-average performance in math and reading is partially due to them not being exposed to recent education changes that have more rigorous standards.

Tennessee was among 13 states that voluntarily participated in the latest National Assessment of Educational Progress (NAEP) test, also known as the nation's report card.

Twelfth-graders in those states were tested last year from January to March.

The average score of students in Tennessee was 282, which was lower than the national average score of 287 for public school students, according to the results released Wednesday.

In math, Tennessee 12th-grade students scored 145, compared to 152 nationally.

Last year, NAEP's assessment of Tennessee students in fourth- and eighth-grade reading and math showed the state leading the nation in academic improvement.

Huffman said those students have been exposed to Common Core education standards and other courses that allow them to take higher levels of subjects like math and science.

"The fourth- and eighth-graders have gone through school most of the way with higher standards in place, and the 12th-graders haven't," Huffman said. "We've got to upgrade the rigor of the classes and make sure that all kids have access ... to rigorous classes."

The commissioner said another factor may be that "elementary and middle schools are just improving faster than high schools."

"We have to take real stock of the fact that our high schools have to deliver a better product," he said.

The latest NAEP results showed only about one-quarter of U.S. high school seniors performed solidly in math, reinforcing concerns that large numbers of students are unprepared for either college or the workplace. One of the main objectives of the Common Core standards - which ha ve been adopted to some degree by 45 states - is to provide students with the critical thinking, problem solving and writing skills needed for college and the workforce.

According to NAEP, almost 4 in 10 students reached the "proficient" level or higher in reading.

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 come from a representative sample of 92,000 public and private school students.

David Driscoll, chairman of the National Assessment Governing Board, which sets policy for the exam, called the stagnation "unacceptable."

"Achievement at this very critical point in a student's life must be improved to ensure success after high school," he said.