Graduating seniors at Alexandria's T.C. Williams High School this year had a 31-point drop in their average SAT score compared with the Class of 2003, school officials said Tuesday.

It was the largest drop in average scores on the college entrance test for any district in the Washington area and marked the first decline in T.C. Williams's SAT scores since 2000. School officials said an increase in the number of test takers, including many with mediocre grades and little test preparation, might explain the drop. T.C. Williams's overall average score was 957.

"It is very hard to compare one class to another when you are talking about two different groups of people who may not have had comparable backgrounds," said Alexandria School Board Chairman Mark O. Wilkoff.

This year, 362 T.C. Williams seniors took the three-hour SAT, compared with 343 last year. Educators said the 6 percent increase in participation was significant, because SAT average scores often decline when more students take the test. The additional test takers are often from families new to the college process and who cannot afford test preparation courses and other methods of ensuring a good score.

T.C. Williams students' average verbal score, 483, was 10 points below the 2003 average. The average math score, 474, was 21 points below last year's. About 40 percent of T.C. Williams students qualify for federally subsidized meals, one of the highest poverty rates in the Washington suburbs, although the school sends several seniors each year to very selective colleges.

SAT scores for Arlington County seniors rose slightly. Seniors who graduated from the county's four high schools this year had an average total score of 1085, five points higher than the 2003 average. Arlington school officials said this was the highest average score for the school system since the SAT scoring system was adjusted in 1995.

"I'm pleased that Arlington students' SAT results have risen this year and have again exceeded both state and national averages," said Arlington Superintendent Robert G. Smith. "I believe the rising scores may be attributed to a variety of factors, particularly the hard work of the students and the support provided them by their families and teachers.

"We hope, as well, that our efforts to encourage greater numbers of students to enroll in rigorous courses and our emphasis on teaching for understanding, as opposed to reducing instruction to that which is tested, have had something to do with the rising scores."

Smith said he was pleased that the average score of African American students, up 29 points from 873 to 902, was the highest in five years. For all test takers in the county, the average verbal score increased two points to 543 and the average math score was up four points to 542.

Many educators and testing experts, including the College Board, which owns the SAT, warn against using SAT averages to measure the academic success of school districts, because the average scores are closely tied to average parental income and what percentage of students choose to take the voluntary tests to qualify for college. The participation rate was 69 percent in Alexandria, compared with the national rate of 48 percent, and undetermined in Arlington.

The national average SAT score this year, which includes private school students, was 1026, unchanged from last year. The Virginia average score was 1024, also unchanged from a year ago.

Kathleen Wills, Arlington's director of planning and evaluation, was one of several testing experts who warned this year of a rapid increase in the percentage of students not marking their race on their SAT registration forms.

The portion of "non-disclosers," as they are called, rose to 38 percent in Arlington and 25 percent nationally last year, and Wills said the trend could make it harder to track minority students' progress on the test.

But the College Board changed its online registration system, and far fewer students skipped the race question this year.

Wills said the portion of non-disclosers dropped to 27 percent in Arlington and 19 percent nationally.