With nostalgia, the teen-age boys at the lunch table at St. Stephen's Episcopal School said they were sorry their successors will have to endure what they have been spared: girls.

They imagined the worst. Instead of discussing homework in the halls of the private Alexandria school, boys will be negotiating dates, said one. School spirit will dwindle and boys will start caring about their clothes, said another. Classes just won't be the same, and the relaxed atmosphere will be lost, they agreed.

"Yeah, like you can't sit down with your zipper open," joked another.

This time next year, St. Stephen's of Alexandria will join other private single-sex elementary school programs in Northern Virginia that, faced with declining enrollment, are going coeducational.

The Washington area historically has had one of the nation's largest concentrations of single-sex religious-affliated schools, and many single-sex secondary schools remain in operation. However, Northern Virginia school officials say that changing demographics have led to an increasing demand for coeducational programs.

St. Stephen's, the last Episcopal Church-affiliated all-boys elementary school in Northern Virginia, will accept girls in grades one through three beginning next fall. Its secondary school will remain male.

Linton Hall in Prince William County, the last all-boys Catholic school in the Arlington Diocese, will accept elementary-grade girls for the first time next year.

Marymount Junior School, an independently run all-girls Catholic school in Arlington that teaches kindergarten through eighth grade, will close so that Marymount "I'd rather have it coeducational because that's what the real world is like."

-- David Miller

University, which this year accepted its first undergraduate men, can expand.

"I think everyone in independent education has got to look at {going coeducational} these days," said the Rev. Christopher Brookfield, dean of the six church schools of the Episcopal Diocese of Virginia. Those schools have experienced a 5 percent drop in enrollment each year for the past five.

St. Stephen's has 534 students this year, against an average of about 550 in the preceding few years.

Marie Powell, superintendent of schools for the Catholic Diocese of Arlington, said local Catholic schools also have been hit by declining enrollment in recent years. In 1980, enrollment in Catholic elementary schools in the diocese was 9,436. This year it was 9,200, despite healthy growth in the Northern Virginia counties it serves.

According to the Bunting and Lyons Directory of Independent Schools, of the 757 private day schools in the country, 627 are coeducational. Of the others, 61 are female and 69 are male. Of the 69 all-male schools, 39 have a Catholic affiliation.

Marcia Sharp, director of the Women's College Coalition, said that the widely held belief that girls enhance the educational milieu at all-boys schools prompted many of those institutions to go coed. "There has been a felt need to bring in girls {to boys' schools}," she said. "The great march toward coeducation was predominantly led by the boys' schools."

Sharp said girls' schools have been more reluctant to go coeducational, in part because of the fear that boys will dominate classroom discussions and command the teachers' attention more easily.

In Northern Virginia, administrators at some all-girls elementary schools said they see a need to retain their single-sex programs.

Such is the position of administrators at St. Agnes School, the sister Episcopal school to St. Stephen's. Next year St. Agnes, which currently has coeducational kindergarten and first and second grades, will be a competitor with St. Stephen's for the first time. With this in mind, shortly after St. Stephen's announced its coeducational plans, St. Agnes shot back with an announcement that it would extend its coed classes to the third grade.

However, school administrators said last week that they do not want to extend coeducation further and that they are confident that the school's mission as an institution for girls will weather the market pressures.

"We have confidence in ourselves. We're academically strong," said Joan G. Ogilvy Holden, headmistress at St. Agnes. "It's like marketing a product. If you're doing it well and your constituency is telling you it's good, you just have to keep doing it."

Founded in 1924, 20 years before St. Stephen's, St. Agnes holds fast to the idea that girls are more likely to succeed academically if they are not burdened with boys in the classroom.

"We have found that, particularly in adolescence, it's important for girls to feel good about themselves, not to feel that they need to be silent," said Diane Dunning, director of admissions at St. Agnes.

Parents and administrators at Marymount Junior School and Marymount University said they are saddened by the turn of events there. "We did fill a unique little niche in Northern Virginia," said Dolores Viehman, president of the junior school's parents association.

Being all-girl "was one of the strengths of the school, and there are none now in the area," said university provost Alice S. Mandanis. "If anything, there is a market for that."

Opinion among the older boys at St. Stephen's was divided about the changes coming to their school.

"I'd rather have it coeducational because that's what the real world is like," said David Miller, 17.

"I think it will be more of a distraction for the boys," said Fred Truslow, 16. "It says something to go to one of the last all-boys schools. It says something special about the boys. Maybe I have old-fashioned views, but I think single-sex education is better than coeducational schools."

The school's headmaster, the Rev. Edwin Ward, said younger students and alumni who have been out in the work world for several years are more supportive of the change than are older students and the recently graduated alumni. Among the latter, he said he has heard the distraction theory before.

"I see that as a sexist orientation that ought to go, that the girls are the sirens who are going to pull the boys off course," he said. "If we're not strong enough academically {to let girls in}, then I would say we didn't have a strong program to begin with."