Down in the art department at Sligo Junior High School in Silver Spring, the students are busy silkscreening "Keep Sligo Open" shirts. Along University Boulevard and Dennis Avenue, bedsheet banners loom over the streets proclaiming the same message.

For the third time in four years, the Montgomery County school administration has placed Sligo on the "hit list" of schools to be closed because of declining enrollment. And, for the third time, the people who live near the school and send their children there have reacted as though the very life of their community is at stake.

Struggles of this sort are taking place at 11 other schools in the county and hundreds of schools across the country as public school administrators try to adjust to the simple fact that there are not as many school-aged children as there were in the baby-boom years.

Nowhere is the battle more intense than at Sligo, a 20-year-old school nestled amid the oak trees and red-brick colonial homes of the Four Corners neighborhood.

Along with the special school-lunch menus, T-shirts and banners, the "Save Sligo" movement has ordered 500 buttons, set up a telephone lobbying bank of volunteers, spread leaflets around the community and organized some 200 parents, teachers and students who will travel by car and bus tonight to the administration building in Rockville for the Board of Education meeting.

There, they will encounter school bureaucrats and politicians who say that they hate to close schools but have to do it to save money. As for Sligo, in the words of school planner George W. Fisher, "It was a tough call, up there with the toughest."

Sligo supporters had gotten off easy with just a modest letter-writing campaign the first time the school was studied for closure in 1975. Last year, encouraged by rallies, more than 300 placard-carrying citizens turned out to see the school board defer school closings until the school system's planners could draw up a comprehensive five-year plan.

When the plan emerged this fall, Sligo was again on the "hit list."

But Save Sligo '79 had started well before the official announcement. "We figured we'd be up for closing again," said PTA President Marion Leach. "When it came time last spring to nominate PTA officers we stayed on. With our experience it would be easier to gear up."

Four weeks ago, Leach's daughter, Jennifer, the ninth grade student government president, was let out of class by a sympathetic English teacher, and along with several other students, spread leaflets around the community.

"My fifth-grade daughter thinks that saving Sligo is part of your life," said Florence Bayard, president of the Four Corners Elementary School PTA. "I told her to get an old shirt to silkscreen but she got a new one because she didn't think it was a oneshot deal. She thought it would become a part of her wardrobe." On the door to the cafeteria, the kitchen staff had posted a special menu: Open Salad, Neighborhood Cookies, and for the 80 percent of Sligo's 863 students who live close enough to walk to school, No Bus Soup.

Closing the school is such a pervasive topic that teachers assigned papers on last year's hearing. Even a bastion of apolitical concerns like the art department is in the thick of it, silk-screening T-shirts that say Keep Sligo Open. On Suport Sligo Day, Assistant Principal Joe Nalton, who as an administrator is technically nonpartisan, had pulled one on, over his tie.

"We're not fighting over bricks, it's the human element that's got us going," said Marion Leach.

Schools are chosen for closing through a complicated identification process that examines, among other things, their racial balance, enrollment figures, distribution percentages to higher schools, and their need for renovation. Although Sligo has fairly stable enrollment projections, comfortably about the 600 minimum enrollment set by the school board, the building was picked mainly because the five rival physical plants of junior highs it was compared with were in better shape.

"Much of the decision is subjective," said Fisher, the planner who made the initial recommendation to close Sligo. "There always has to be a subjective judgment of objective criteria."

Even Sligo supporters concede that a junior high school be closed in the area west of the Georgia Avenue corridor, which has shown some of the most severe enrollment declines in the country.

But the objections to it being Sligo pour forth with a sense of injustice. The enrollment there is increasing slightly, parents say. Why single out Sligo? The building is in pretty good shape, and the community has already been hit hard by six elementary school closings.