High school seniors are performing worse overall on some national tests than they did in the previous decade, even though they are receiving significantly higher grades and taking what seem to be more rigorous courses, according to government data released yesterday.

The mismatch between stronger transcripts and weak test scores on the National Assessment of Educational Progress, often called the nation's report card, resonated in the Washington area and elsewhere. Some seized upon the findings as evidence of grade inflation and the dumbing-down of courses. The findings also prompted renewed calls for tough national standards and the expansion of the federal No Child Left Behind law.

"We have our work cut out for us," Education Secretary Margaret Spellings said in a statement. "If, in fact, our high school students are taking more challenging courses and earning higher grades, we should be seeing greater gains in test scores."

About 35 percent of 12th-graders tested in 2005 scored proficient or better in reading -- the lowest percentage since the test was launched in 1992, the new data showed. And less than a quarter of seniors scored at least proficient on a new version of the math test; officials called those results disappointing but said they could not be compared to past scores. In addition, a previous report found that 18 percent of seniors in 2005 scored at least proficient in science, down from 21 percent in 1996.

At the same time, the average high school grade-point average rose from 2.68 in 1990 (about a B-minus) to 2.98 in 2005 (about a B), according to a study of transcripts from graduating seniors. The study also found that the percentage of graduating seniors who completed a standard or mid-level course of study rose from 35 to 58 percent in that time; meanwhile, the percentage who took the highest-level curriculum doubled, to 10 percent.

"The core problem is that course titles don't really signal what is taught in the course and grades don't signal what a kid has learned," said Kati Haycock, president of the Education Trust, a D.C.-based nonprofit group that supports No Child Left Behind. She added hyperbolically, "What we're going to end up with is the high school valedictorian who can't write three paragraphs."

Some experts say these educational mirages, which obscure low student achievement with inflated grades and tough-sounding class titles, disproportionately harm poor and minority students.

A visit to two ninth-grade English classes in Prince George's County this week showed that instruction can vary immensely even in classrooms -- just 15 miles apart -- that share the same champagne-colored textbook, the same course title and the same syllabus.

In Room 101 at Bowie High, a racially diverse school in one of the county's more affluent areas, the assignment was: Compare and contrast the themes of disillusionment, poverty and frustration in George Orwell's "Animal Farm" and the poems of Langston Hughes.

In Room 31 at Suitland High, which has more poor and black students, the assignment was: What are your immediate goals? How would you feel if no one close to you supported you in reaching your goals?

The teacher at Suitland, R'Chelle L. Mullins, walked around the classroom and repeated the assignment several times to the students, some of whose heads were slumped on their desks. "What are your immediate goals?" she asked one boy again.

"To pass the ninth grade," he finally answered.

After class, Mullins said she had "stuck very close to the curriculum" and "was doing exactly what the county wants me to do." But when told of the more complicated questions asked in the Bowie High class, Mullins acknowledged that she sometimes modifies assignments based on the background of her students.

Mullins, 24, who began teaching two years ago because she wanted to help underprivileged children, said she had "a different caliber" of students in her classroom. "Not to dumb my kids down," she added. "I hate the bad reputation that they get, and I don't think it's fair at all. . . . Not to pass the blame, but some of these kids should never have been allowed to graduate middle school."

County Superintendent John E. Deasy said he is working hard to reduce inequities among schools and cited uneven teacher quality as a key issue. He said that the county curriculum has been standardized and that the challenge now is to ensure an equal level of instruction in every classroom by investing in teacher training and increasing the number of Advancement Placement courses.

"This is the civil rights issue of our time," Deasy said.

The potential for grade and course-title inflation is not confined to low-performing schools. Julie Greenberg, a math teacher at Montgomery Blair High School in Silver Spring, said she was under such pressure to raise grades that she used to keep two sets of books in her statistics class: one for the grades students deserved and one for the grades that appeared on report cards.

"If a teacher were to really grade students on their true level of mastery, there would be such extraordinary levels of failure that it would not be tolerated, so most teachers don't do that," she said.

At a news conference yesterday near Capitol Hill, education experts expressed concern that white and Asian students continue to score consistently higher than black and Hispanic students in all subjects. They also said the overall discrepancy between the test scores and transcripts deserves close examination. Darvin M. Winick, chairman of the National Assessment Governing Board, which oversaw the exams and the transcript study, called the gap "very suspicious."

"For all of our talk of the achievement gap amongst subgroups of students, a larger problem may be an instructional gap or a rigor gap," said David W. Gordon, superintendent of Sacramento County schools in California. "There's a disconnect between what we want and expect our 12th-grade students to know and do and what our schools are actually delivering through instruction in the classroom."

Lawmakers said the low test scores would reinvigorate the debate over high school reform as Congress considers the renewal of No Child Left Behind.

Rep. George Miller (D-Calif.), chairman of the House Education and Labor Committee, said "disappointing" results underscore the need to recruit first-rate teachers to low-performing schools.