Prince William County students maintained or slightly improved their standardized test scores this year compared with a year ago, continuing an upward trend that began earlier this decade, school officials said yesterday.

For the sixth consecutive year, students in the county's 52 public schools equaled or posted their best scores on achievement and skills tests developed by Science Research Associates and administered nationwide.

"As you look back over a five- or six-year period and see consistent growth, that is a good sign," said Kristy Larson, a spokeswoman for the 38,000-student Prince William school district, second largest in Northern Virginia and fourth largest in the state.

Virginia requires all fourth, eighth and 11th graders to take the SRA reading, math and English usage tests. Prince William also gives the test in grades two and six.

While Prince William students' scores exceeded the national norm, or 50th percentile, their composite scores ranged from six to 13 percentile points lower than their counterparts in neighboring Fairfax County, which is often used as a measuring stick for the caliber of education in Prince William.

The composite scores, which combine math, reading and language scores, were in the 77th percentile for second-grade pupils in Prince William, meaning that they scored better than 77 percent of students nationally. Prince William second graders scored in the 75th percentile last year and in the 63rd percentile in the 1981-82 school year.

Fourth graders scored in the 74th percentile this year, up one point from last year and nine points since 1981-82.

Sixth graders scored in the 70th percentile for the second straight year, compared with the 64th percentile six years ago.

Eighth and 11th graders scored in the 67th and 69th percentiles, respectively, this year compared with 64th and 60th in 1981-82. Last year eighth graders scored in the 66th percentile, and 11th graders in the 68th.

Second graders recorded both the highest and lowest scores countywide: the lowest was in reading, at the 62nd percentile; the highest was in math, 80th percentile, which represented a five-point gain from last year, the biggest single jump recorded.

School officials, in releasing the test results, noted that variations of one or two points from year to year are not significant, but long-term gains are.

"It is steady progression that is most important to us," said William Cox, the county's associate superintendent of instruction.

School administrators said the SRA test is one of several criteria used to evaluate the quality of instruction in Prince William, a fast-growing county 25 miles from Washington that in recent years has seen an influx of middle-class residents with high expectations of their public schools.

In recent years the SRA and other standardized tests have received growing attention, not only from parents but also from business and industry leaders considering moving their operations to the county, according to Acting School Superintendent Jeff Bloomer.

The county is trying to expand its commercial tax base, now about 14 percent. All the groups ask "How are your SAT scores? How are your SRA scores?" Bloomer said yesterday. "People are not willing to come where education is not a priority."