Prince William County school officials this fall will begin mapping massive boundary changes in an attempt to balance lopsided enrollments that have led to severely crowded schools in some parts of the county and half-empty buildings in other areas.

Superintendent Richard Johnson plans to go before the school board in November with several options to remedy what he calls the "patchwork boundaries." Meanwhile, a school board-appointed citizens' advisory committee is expected to complete by September an enrollment study that could lead to major boundary shifts next fall.

A major factor in the enrollment problems, according to school officials, is uneven development in the county. Although western Prince William generally has grown more rapidly than the eastern part of the county, in recent years there have been building booms in parts of eastern Prince William as well.

Those problems are particularly apparent at Potomac High School in eastern Prince William. When classes resume Aug. 30, school officials say they expect 1,900 students at Potomac, built three years ago to accommodate 1,500 students. Faced with parental protest over the possibility of busing some Potomac students as far as 18 miles to the half-empty Osbourn High School, the school board last year agreed to the overcrowding at Potomac for the coming academic year, hoping the problem would be remedied by boundary changes in 1983.

Scrambling to cope with the overcrowding, Potomac officials have added a fourth lunch hour (at 10:10 a.m.), divided several rooms to add seven classrooms and juggled schedules so every bit of space will be used constantly.

"I want to know why this happened. Why didn't they have enough foresight to plan ahead?" said Buddy Biscardo, student body president at Potomac. "We will deal with the overcrowding. . . . But I'm not thrilled at the thought of eating lunch at 10 in the morning."

Last year, when enrollment at Potomac was 1,400, students already were complaining that the school was crowded. Some students said hallways were so jammed they often worried about making it from class to class in five minutes.

"My son told me to go up to the school just to see how bad it was last year getting through the halls," said school board chairman Gerard Cleary, whose 16-year-old son attends Potomac. "He says, 'Dad, give us room to breathe.' We've given a citizens' committee one year to evaluate this problem. Something has got to be done."

When Superintendent Johnson last year suggested relieving the overcrowding at Potomac by shifting some Montclair area students to Osbourn, which has room for 2,300 students, parents complained that many students would have to ride the bus half an hour each way. After those complaints, the school board rejected Johnson's suggested boundary change, just as it had turned down his proposal six years ago to build Potomac to a 2,000-student capacity. At the time, school board members said they were fed up with large, "super high schools."

"Housing of students is always going to be an issue," Johnson said recently. "We have many boundaries that are rather patchwork in nature so it's time to look at a comprehensive boundary realignment a year away. This is the first time in 15 years that the county has had a chance to look hard at the boundaries and what evolved after all those years of development."

The tight housing market has slowed development in Prince William, stablizing enrollment at 35,000 students in the 49 county schools the last two years. Last fall, the school system went to a traditional nine-month calendar, instead of the year-round schedules that parts of the system had been using the past 10 years. Construction has started on a new elementary school scheduled to open in Lake Ridge next fall.

"Look around and all the districts are closing schools because they didn't go to year-round schools when we did," school board chairman Cleary said. "We bit the bullet and went on year-round; we didn't have that massive building in the '60s and '70s. Finally it just got too costly to continue year-round school. We saw that we could go on a traditional school schedule without massive building."

Before Manassas and Manassas Park pulled out of the county school system six years ago, enrollment in Prince William schools had peaked in 1975 at 39,000 students.

This fall, Manassas school officials expect about 150 new students, bringing enrollment to 3,700. Manassas Park officials expect about 1,500 students, almost the same as last year. School opens Aug. 30 in Manassas and Manassas Park.

In Manassas Park, school and city officials are considering shifting students from Connor Elementary to Independence. If the proposal is approved, it would be the second shift for the 375 students, who moved to Connor last year after the school board closed Independence. Since then, however, the school system has not been able to sell the Independence building, and city officials recently approached the school district about taking over Connor for city offices.

"Independence is in a residential area with easy community access and Connor is in a more industrial area that could easily be converted to either city or private offices," said Superindent Gary Smith. "We should know within 30 days, after public discussions, what will be done. Then the shift could be done over Christmas vacation." A public hearing on the issue is expected to be set for later this month.

Unlike Manassas Park, Prince William has not had to close any schools and even avoided laying off teachers this year. The current school budget in Prince William is $112 million, $14.5 million more than last year. State aid increased $3.9 million, while federal aid increased $556,000, school officials said. Still, Prince William officials warn, the schools don't have money to burn.

"All in all it will still be a tight budget year," said school board chairman Cleary, "even though no teachers or programs were cut. The main thing we're focusing on this next year is drawing the boundary lines so they hold for five or 10 years down the road. No doubt about it, this boundary problem is the most volatile one we face, especially at Potomac High."