As Fairfax County elementary schools bulge at the seams, educators and parents are trying to resolve differences about where to put a burgeoning student population.

To cope with rapid growth in the county's western section, school officials are drawing new boundary lines for allocating students between existing schools and the four new elementary schools that are to open next fall.

Parents, meanwhile, are concerned that the school district changes will interrupt their children's learning and send them away from familiar surroundings.

Dr. Roger W. Webb, director of the school system's Office of Facilities Planning Services, described the situation as one of "feast or famine" because the new schools will relieve the currently overcrowded schools. But he admitted that the county is only "playing catch-up," because growth in Fairfax County is continuous.

The School Board is being asked to approve boundary changes affecting the Armstrong, Salisbury, Sully and Union Mill elementary school sites in the western part of the county, all to open in 1986-87. Other boundary changes next year would affect students at the Cheney and Markham elementary schools in the Fort Belvoir area, and Gunston and Lorton elementary schools in the Lorton area.

Two administrative boundary changes, affecting only future students and not requiring a public hearing, also are awaiting School Board approval. They involve the Cherry Run and Cardinal Forest elementary schools and Oakton and Chantilly high schools.

At the heart of one dispute is the already overcrowded Herndon Elementary School. A look at the strains there and the attempts by school officials and parents to overcome the problems gives a sense of how change is taking place across the fast-growing county.

Because of excessive enrollment at Herndon, county school officials proposed moving 225 students to the new Armstrong area school, which is to open next August. Students tapped for the change were largely from the Kingstream and Crestbrook neighborhoods in the Dranesville Road area, north of the Herndon town line.

Parents opposed the plan because school officials predict Armstrong will be at capacity in three years and the same students then would have to move to another new school, to be located in the Hiddenbrook area and scheduled to open for the 1988-89 school year.

In response, school officials revised their boundary line proposal for the district so that only 45 students would be moved from Herndon to Armstrong in the fall. Those affected under the current proposal would be residents of the Stuart Woods Apartments, Crestview Town Homes and Herndon Estates.

The recommended capacity at Herndon is 830 students. The school had 965 this year, according to September enrollment figures, and is expected to have 1,076 in September 1986 if there are no boundary adjustments, said Webb.

Under the revised proposal, the school would have about 1,000 students for the 1986-87 school year, Webb said. This includes 25 new students expected to move into the area but who would attend Forestville Elementary School instead of Herndon.

"The intent is to provide some relief to Herndon in the next couple of years, in anticipation that a new school will be constructed at the Hiddenbrook site," said Webb.

President of the Herndon Elementary Parent-Teachers Association Suzanne Flegal said she does not understand why county educators did not anticipate the growth and head off the problems earlier.

Flegal expressed satisfaction with the new plan to keep most students at Herndon until the Hiddenbrook school is completed, and said: "We're willing to put up with overcrowding in our school rather than have our children go to three different elementary schools."

Herndon Principal Wayne Chester approves of the plan and said his staff has been managing well under the overcrowded conditions.

"It'll continue to be a challenge, but we can take care of it," he said. "We've learned a little self-discipline that we didn't know before . . . . We go to use the playground when we're scheduled to use it, or someone else will be using it."

Chester added: "We're delighted, for the most part. We're delighted we'll have the youngest in our community in our school. And when they change schools, it'll be their community school then. That's what educators want."

As part of the effort to appease families, school officials will hold meetings in January with all Fairfax County communities affected by proposed boundary changes. The Fairfax County School Board will hold a public hearing Feb. 10, to elicit citizens' comments on the final boundary proposals.

After the superintendent presents the final recommendations Feb. 13, the School Board will act on it. That vote is scheduled for Feb. 27.