SAT scores for the Class of 2015 were the lowest since the tests’ overhaul in 2005. (Joe Raedle/Getty Images)

Scores on the SAT have sunk to the lowest level since the college admission test was overhauled in 2005, adding to worries about student performance in the nation’s high schools.

The average score for the Class of 2015 was 1490 out of a maximum 2400, the College Board reported Thursday. That was down 7 points from the previous class’s mark and was the lowest composite score of the past decade. There were declines of at least 2 points on all three sections of the test — critical reading, math and writing.

The steady decline in SAT scores and generally stagnant results from high schools on federal tests and other measures reflect a troubling shortcoming of education-reform efforts. The test results show that gains in reading and math in elementary grades haven’t led to broad improvement in high schools, experts say. That means several hundred thousand teenagers, especially those who grew up poor, are leaving school every year unready for college.

“Why is education reform hitting a wall in high school?” asked Michael J. Petrilli, president of the Thomas B. Fordham Institute, a think tank. “You see this in all kinds of evidence. Kids don’t make a whole lot of gains once they’re in high school. It certainly should raise an alarm.”

It is difficult to pinpoint a reason for the decline in SAT scores, but educators cite a host of enduring challenges in the quest to lift high school achievement. Among them are poverty, language barriers, low levels of parental education and social ills that plague many urban neighborhoods.

Cyndie Schmeiser — chief of assessment for the College Board, which owns the SAT — said she is concerned because the share of students prepared for college has stagnated for five years. Close to 42 percent of students who took the SAT reached a score of at least 1550, a benchmark for college and career readiness. The share was far lower for Hispanic students (23 percent) and African Americans (16 percent).

“Simply doing the same things we have been doing is not going to improve these numbers,” Schmeiser said in a statement. “This is a call to action to do something different to propel more students to readiness.”

Schmeiser cautioned against “overinterpreting small fluctuations” in average scores from year to year.

Caveats abound when SAT scores are released. The students who take it are in most cases a self-selected sample, motivated to endure a grueling exercise of 3 hours and 45 minutes on a Saturday. (The test is offered during school days in all public high schools in the District of Columbia and a handful of states.)

Some students take the SAT two or three times. Scores also track closely with family income, rising with affluence, so annual variations in who takes it can swing the results. That makes comparisons of scores among schools, school districts or states problematic. The lower the participation, generally, the higher the scores.

North Dakota, for example, has a seemingly stellar score: 1791, about 300 points above the overall average. But just 134 North Dakotans in the Class of 2015 took the SAT; nearly all students in that state take the rival ACT exam. By contrast, the SAT average in Idaho is 1372. But nearly every graduate in Idaho this year — 17,695 in all — took the SAT.

Since 2005, the SAT has included a writing section worth up to 800 points. That will change in March, when the College Board rolls out a new version that makes the essay optional, does not penalize guessing and contains fewer obscure vocabulary words. The perfect score will snap back to what many parents and teachers remember — 1600.

Through the redesign, Schmeiser said, the College Board aims to “deliver opportunities for students to succeed in college and careers. It will take time to improve these numbers, but we’re deeply committed to making progress.” She cited a partnership with the nonprofit Khan Academy to offer free online tutorials for students preparing for the new SAT. College Board officials are hoping to level the playing field for students who can’t afford expensive test-prep classes.

About 1.7 million students in the Class of 2015 took the SAT, up 1.6 percent from the previous class. That total includes people taking the test overseas. The number tested in the 50 states and the District was roughly 1.5 million. The total who took the ACT in the United States was about 1.9 million.

Many students take both tests and submit the one that gives them the best shot at college admission. But the SAT remains more popular in the Washington region.

In the nation’s capital, which has offered the SAT for free for juniors and seniors since 2013, about 4,700 students from the 2015 class took the exam. Those from private schools, with many educational advantages, reached an average score above 1800. Those from public schools in the District reached an average score of 1139, a 24-point gain from the previous class. The public-private gap illustrates how scores correlate with wealth.

D.C. Schools Chancellor Kaya Henderson is seeking to rejuvenate many of the city’s high schools this year with an expanded curriculum in advanced courses, arts and other electives. She has dubbed it “The Year of the High School.” Brian Pick, chief of teaching and learning for D.C. Public Schools, said the school system is committed to ensuring that students are prepared for college and careers.

“Student success is at the heart of our work, and we want to make sure our students have equitable access to this test and [are] being prepared to do well in college, and in life,” Pick said.

In Maryland and Virginia, participation in the SAT was almost unchanged. More than 7 of 10 graduates in each state took the test. Maryland’s average score was 1462, counting public and private students, and Virginia’s was 1533.

Virginia touted the scores of its public school students, which rose a point in math, to 513, and 2 points in writing, to 495. The reading score, 515, was unchanged. The results “provide further confirmation that the higher expectations for learning and achievement adopted by the Board of Education are resulting in better-prepared graduates,” Billy K. Cannaday Jr., president of the Virginia board, said in a statement.

Maryland’s public school students scored an average of 1434 on the SAT, down 4 points. State officials pointed out that participation in the ACT is rising, perhaps signaling a market shift.

“SAT scores have flattened in Maryland,” said William Reinhard, a spokesman for the Maryland State Department of Education. “There does seem to be a conversation going on among families and high school students on whether to shift to another exam.”

Michael Alison Chandler and Moriah Balingit contributed to this report.