For nearly a decade, school officials in Loudoun and Prince William counties have boosted teacher salaries, added courses and built shiny science labs in a bid to transform once-rural schools into modern suburban ones more like those in neighboring Fairfax.

But now, confronted with the worst budget crisis in memory, school officials and parents in the outer suburbs fear that cuts in programs and teacher salaries will mean their schools will lose ground.

Proposed cuts in Loudoun "will turn back the clock of progress," Claire Mann, a parent, told about 500 parents and teachers at a School Board budget hearing in Leesburg last week. "Don't make our school system one that nobody wants. It may become the county nobody wants."

School systems throughout the Washington area are struggling with budget problems because of a downturn in local government revenue. In Northern Virginia, all school systems face additional cuts in state school aid next year as Richmond tries to make up its own budget shortfall. But administrators in Loudoun, which stands to lose $2.6 million, and Prince William, which is looking at a $5.5 million loss, say their schools are more vulnerable than others.

Neither county has the broad tax base of a Fairfax to help temper state cuts, administrators say, and both have been hit hard by the sagging local real estate market.

"We're in a competitive marketplace . . . . You want to make sure the students that come out of Loudoun are as well-equipped as students in any suburban system," said Harry Bibb, acting superintendent in Loudoun. "Once you lose something, it's almost impossible to get it back."

With Loudoun County facing a $31 million revenue shortfall next year, and cuts promised in state aid, Bibb has proposed increasing spending for the system's 14,600 students by just over 3 percent next year and freezing the pay of all employees. The proposed $95.5 million operating budget comes with an appendix outlining $11.7 million in further trims, including teaching slots, programs for gifted students, even supplies. Cuts of at least $7 million are almost a certainty, school officials said.

Prince William officials expect that their budget, to be submitted March 6, will be smaller than the current $231 million operating budget, said Robert Ferrebee, associate superintendent of Prince William schools. The total budget is expected to shrink next year by as much as $2.5 million, he said.

"It's the first time it has ever happened to us," Ferrebee said. "This really is gloom and doom."

Prince William Superintendent Edward L. Kelly, who has worked for three years to make his schools and teacher salaries more competitive, said: "When you're still climbing, it's important that you make sure you continue that progress. If you get slowed down or stopped, it's hard to get going again."

Although Fairfax faces cuts in state aid of $21.7 million next year, and plans to keep much spending flat, its proposed $903.7 million operating budget does allow for seniority raises for teachers. No cuts in academic programs for the 130,000-student system are expected, said Charles Woodruff, the county budget director.

Prince William will have to freeze teacher pay or cut some academic programs for its 42,000 students to balance the budget, a choice the district has never had to make.

The list of 40 possible cuts in Loudoun includes: Cutting participation in the regional Thomas Jefferson High School for Science and Technology in Fairfax, which serves about 60 Loudoun students, for a savings of $514,000. Shrinking the county's gifted program by cutting weekly classes for students in grades 4 through 8 and a program to identify gifted children in grades 1 through 3, for a savings of $396,000. Raising average class size from 22.5 pupils to more than 25 by cutting 51 teaching positions, for a savings of more than $1 million. The cuts would mean combining some grades in the small elementary schools, cutting some high school classes with low enrollment, such as computer science, or offering some classes such as psychology or sociology every other year. Cutting 30-minute, twice-weekly music instruction by trained music teachers for elementary school children. Classroom teachers would teach music once a week for 40 minutes, for a savings of $216,000.

If implemented, the additional cuts "have the shocking potential for decimating our school district," said Kay Franklin, president of the Loudoun Education Association, which represents about 800 teachers.

The Prince William and Loudoun school systems have taken steps in the past decade to become more competitive, raising their per-pupil expenditures by 131 percent and 128 percent, respectively, and boosting teacher salaries. A beginning teacher in Prince William now makes $300 more annually than a first-year teacher in Fairfax.

In addition, Prince William has added a seven-period day for every high school student, and Loudoun has added elective foreign language classes for seventh-graders.

"There has been a lot of progress in the last few years," said Eugene Carr, president of the Loudoun County Chamber of Commerce. "Taking a step backward is something Loudoun can ill afford to do."

Big budgets don't automatically mean preeminence in test scores. Last year, Loudoun students scored virtually as well as Fairfax students, even though Fairfax County spent more per pupil. Still, many suburban parents want to see their schools spend as much on each student as other schools in the region.

Templar Titus, the father of a Loudoun County High School student, said that as Loudoun becomes a more integral part of the Washington suburbs and competition to get into the best colleges grows more intense, the county can't afford to cut back on academic programs.

"Any program that affects the current classroom teaching level should be retained at all costs," he said at a recent budget hearing. "To eliminate or dilute programs and positions such as summer school, art, music, athletics, driver's education and elementary school principals would drastically affect the quality we need to keep our kids in the forefront."