Three thousand new students are expected to attend Fairfax County's 187 schools when classes begin Tuesday, bringing enrollment in the metropolitan area's largest system to nearly 131,000.

There will be some changes that students and their parents will notice, including a new elementary school in the western part of the county, alterations in the elementary school math curriculum and expansion of a program for gifted students in kindergarten through second grade. Numerous schools, including at least eight high schools, will have new principals.

The two major issues this year, however, will be teacher pay-for-performance and school boundary changes, both of which will have large long-term impact, but little immediate effect, on what students learn in the classroom.

The Fairfax school system, the nation's 10th largest, is overhauling its teacher salary structure by phasing in a tougher on-the-job evaluation system that will be the basis for all pay raises by the 1989-90 school year.

Last year, the system was tested in eight county schools with mixed results. This year, slightly revised, it will be used in all schools for 40 percent of the county teaching force. Next year, the remaining teachers are to be evaluated with the new method.

Students whose teachers are being evaluated may notice more observers in the classroom, and teachers who are assisting with evaluating their peers may be absent more than usual.

The county's major teachers union, the Fairfax Education Association, agreed to the evaluation system in return for pay raises totaling 30 percent over three years. Cost of the plan is more than $110 million over three years.

Fairfax County School Superintendent Robert R. Spillane is an ardent promoter of pay-for-performance, arguing that it will improve instruction, keep the best teachers in the classroom andconvince the public that the school system is cracking down on incompetent teachers.

But Fairfax County has not been exempt from problems plaguing merit pay plans across the country. Teacher morale has dropped because of the evaluations, some teachers complain the ratings are unfair, and the FEA has announced it will sue the school system because it froze the pay of teachers who received less than satisfactory ratings last year.

Some community groups protested that the standards may have been set too low, noting that two-thirds of the teachers who applied for merit pay at the eight pilot schools last year received ratings high enough to qualify for it.

Some county officials question the school system's failure to provide long-term estimates of the merit pay program's price tag, saying it will add hundreds of millions of dollars in retirement plan costs.

The other contentious issue facing the schools this year will be the boundary changes needed to accommodate a new high school and five new elementary schools that will open in 1988-89. As many as four dozen elementary school communities will be affected by the changes, said Alton Hlavin, an assistant school superintendent.

The new high school, Braddock Park, is on Braddock Road between Clifton and Fairfax, and likely will draw students from areas now served by Chantilly and Fairfax high schools. Officials are considering a controversial plan to use it for grades seven through 10 and phase in upper grades later. The school has a capacity of 2,000 students but will enroll perhaps half that in its first year.

The five elementary schools are Hidden Brook in Herndon, Moneys Corner in outer Reston, Sangster Branch along Pohick Road, Bonnie Brae on Sideburn Road, and Silverbrook on Silverbrook Road, which will replace Lorton Elementary School. All school names are site names only; the School Board will vote on final names next spring.

Three more elementary schools are due to open the following year: Willow Springs along Braddock Road, Virginia Run in Centreville, and Saratoga in the Newington area. Hlavin said his staff would probably recommend that boundaries be set for those schools this year so that parents know far in advance where their children will attend.

Boundary changes are emotional issues in Fairfax County, particularly when they involve high schools, which are the center of community life for many residents. School system officials say they will consult area civic groups on their plans in the fall, propose specifics to the School Board at the Dec. 17 board meeting, and hold a School Board work session on the proposals Jan. 7. Final recommendations are to go to the board Feb. 11, with a public hearing scheduled for Feb. 22 and School Board action Feb. 25.

Only one new school building opens this year, Lees Corner near Chantilly. It has a unique design with a bell tower and will house 581 students, officials said. The school itself opened last fall, but its students attended the Neil Armstrong Elementary School in Reston during the past school year.

Other changes in the schools this year will include new mathematics textbooks for grades one and two, the first phase of a new elementary math curriculum that emphasizes problem-solving rather than rote computation. Some schools will add new textbooks in the other elementary grades this year; others will phase them in over three years.

Parents should see the results in the homework assigned to their children, said Marjorie McClurg, county mathematics coordinator. When the county revised the intermediate school curriculum in a similar way several years ago, some parents complained that they couldn't do the math problems, but McClurg said no such gripes are anticipated with the simpler elementary school questions.

A new algebra program will be introduced in county high schools this year, and writing skills will be heavily emphasized in county secondary schools, according to educators.

"The major thing students will notice is more use of computers," said Mary Anne Lecos, the assistant superintendent in charge of instruction. Each intermediate and high school will implement new uses of computers in two of three curriculum areas, science, math and writing, this year.

This year will see the expansion of a gifted and talented student program for kindergarten through second grade to all county schools. The program has been phased in over three years; this year, 72 additional schools will offer it.