Sister Doris Healy, 68, who attended Immaculata Preparatory School as a girl and now works in its office, cried briefly yesterday as she talked about the school being closed.

"If we had more sisters, we could go on," she said, "but there aren't enough of us. It takes a great deal to be a nun . The girls say they love us, but each one says, 'Let her join, not me. It's a good thing, but not for me.' "

Early yesterday morning, the Sisters of Providence, the order of Catholic nuns that operates Immaculata Prep, called their students into an assembly to announce officially what most already knew. In June 1986 the school at Tenley Circle NW would be sold for $7.6 million to American University.

Sister Ann Doherty, the order's general superior who came from its headquarters in Indiana for the announcement, said the money was needed to care for the order's aging members, more than a third of whom are over 70 years old.

Very few young women are joining, she said, just five this year nationwide and only one a year ago. Those nuns who work -- now only half of the order's 955 members -- can no longer pay for the medical care and support of those who are aged and infirm.

But when she suggested that girls in the audience might become nuns themselves someday, some snickered loudly and laughed, even though a few minutes before they had been crying about the shut-down of their school.

Later, senior Christy Sobota explained, "I love this school. It's a tradition. The nuns are great. We are their whole lives."

But she added, "Who wants to be a nun in this day and age? . . . I like freedom."

Sister Michaela Galvin, the principal, said no girl from Immaculata had joined the order in 15 years.

"It's a fact of life -- thoughout the country, not just here," she said, "through all the religious communities, not just ours . . . . We have to face it. Very few are coming behind us to do what we do now . . . . We have to have a trust fund raised by selling property to care for us in the future. But people find that hard to understand."

The 8.2 acre campus includes Immaculata Dunblane, a grammar school and junior high, as well as Immaculata Prep, a senior high. The two schools together enroll about 560 students, all of them girls. Both were established on the property, at Wisconsin and Nebraska avenues NW, in 1905 and will be closed in June 1986 when the sale is expected to be completed.

Yesterday, there were hand-lettered signs on the fence, reading "We Love Our School," and "Heck No! We won't Go!" Many of the students cried and then, as a television film crew came, they cheered for their school defiantly: "IPS, IPS, Long Live IPS."

A group of parents seemed upset. Some complained that they hadn't been told earlier about the impending sale. Some said they might try to raise funds to establish a new school at a different location, but with most of the same staff.

"There are many places where children can get an education," said Dee Dillon of Gaithersburg, who is president of the Mothers Club, "but here they get more than an education . . . . We should have been told before this. Maybe we could have helped."

About two-thirds of Immaculata's 455 students come from Maryland, most of them from Montgomery County, but 45 come from Prince George's and one from Anne Arundel. Only 31 percent live in the District of Columbia.

About 16 percent are minorities; almost a fourth are daughters, sisters, or nieces of Immaculata graduates. About 11 percent receive financial aid to pay the $2,500 annual tuition -- a fee that is far less than at most of the area's other private schools, where tuitions this year range up to $5,500.

In 1978, Immaculata College, a two-year women's college that occupied part of the property, was closed down because of declining enrollment and rising costs. Enrollment at the other two schools remains high with more than two students applying for each one accepted.

The old college dormitory and some of its classrooms have been leased to American University, whose main campus, at Massachusetts and Nebraska avenues NW, is just under a mile away.

AU President Richard Berendzen said his school had not yet decided how to use the property and its four buildings, but he said it would relieve "inadequate facilities." The site is particularly valuable, he said, because it is just a block from the Tenley Metro station that opened in late August. The purchase price was determined after an appraisal was done, Sister Ann Doherty said.

"We were waiting for the subway for years," Sister Michaela said yesterday, "and now just as the subway opens, we announce that we have to close. But I'm not sure how long the school could go on."

From 1960 until last spring, the principal of the school was Sister Mary Clare Fritsch, who died in June at age 79.

Sister Michaela said Sister Mary Clare had transformed it from a "select finishing school," called Immaculata Seminary for Young Ladies, into a rigorous college preparatory school, which took its present name in 1965.

Even though enrollment grew and programs flourished, the number of nuns on the faculty has dropped to just five out of 40 teachers.

Since 1970, the Sisters of Providence has sold seven of its schools around the country. Besides Immaculata, only two are left -- in Chicago and New Hampshire.

Last January, Sister Mary Clare approved changing the Immaculata school uniform -- to maroon and tan for freshmen this fall instead of the previous all-blue.

"She was aware of the possibilities," Sister Michaela said. "But she never, never believed Immaculata would close. She was an eternal optimist, but this just has to be done."