Prince William County students achieved their best scores ever on standardized achievement tests for three grade levels, according to test results released yesterday.

Students in the second, sixth and 11th grades turned in the highest scores recorded for those grade levels since the school began administering the Science Research Associates achievement test, officials said.

The results, most of which exceeded the national average, continued a five-year pattern of steadily increasing scores in mathematics, reading and language, according to school spokeswoman Kristy Larson.

"This is one more indication that Prince William County schools are continuing in the direction we want them to go," said Richard W. Johnson, superintendent of the school system, which is Virginia's third largest.

Despite the improvement, Prince William County students still lag behind Fairfax and Arlington counties and some other Northern Virginia school systems by an average of about 10 points on most of their test scores.

The composite score, which combines scores on all three sections of the test, reached the 75th percentile for second grade pupils in Prince William.

That result means that the youngsters scored at least as well as 75 percent of the pupils taking the test nationwide.

Sixth graders scored at the 70th percentile, and 11th graders at the 68th percentile.

The overall 66th percentile score for eighth graders was down 1 percentage point from last year, and the total score for fourth graders remained unchanged from last year's 73rd percentile.

The SRA tests are administered widely across the country, with the 50th percentile representing the score exceeded by half the students tested.

Virginia mandates SRA testing in grades four, eight and 11. Prince William County voluntarily gives the test in grades two and six.

School officials said the results indicate that the school system remains on a path of slow but steady improvement of its student test scores. Johnson credited the improvement to a back-to-basics approach emphasized by the system in recent years.

Yet Johnson acknowledged another factor influencing recent improvements: the increasingly sophisticated composition of Prince William households as the county enters the third decade of its transformation from a largely rural area to a suburban locality.

"The demographics of this county have changed tremendously, and aspirations people have for their children have changed with them," Johnson said.