After 81 years and about 3,800 graduates, Immaculata Preparatory School in Northwest Washington graduated its last class yesterday.

There was a red rose for each of the 62 graduates, most of whom smiled happily as they left the ceremony in nearby St. Ann's Church. Some of the teachers seemed tearful and subdued.

Meanwhile, five miles away in Rockville it was a normal day, teachers said, at Immaculata College High School. That institution was founded by parents after the Sisters of Providence, an order of Catholic nuns, announced last year that they would close the Immaculata school that they had run on Wisconsin Avenue at Tenley Circle NW.

The sisters sold the 8.2-acre campus, occupied by Immaculata and Dunblane, a grammar school and junior high, to American University for $7.6 million. They said the sale was necessary to provide funds for the retirement and medical care of the order's aging members, more than half of whom are over age 65.

"There's a lot of heartache," said Sister Michaela Galvin, Immaculata's principal. "But it had to be done. We've been here for many, many years. But there's no other source of income to care for the aging sisters."

"It makes me really sad that the school is closing," said graduate Susan Ingraham of Bethesda, editor of the school's last yearbook. "But there just aren't enough younger sisters coming in. I guess they could call the school Immaculata. But it wouldn't be Immaculata without the nuns ."

No girl from Immaculata school has joined the order in 15 years. Membership in Catholic religious orders has declined sharply throughout the country. This year the Sisters of Providence, down to 910 members compared to 955 a year ago, had only one new member taking vows, according to Sister Anne Doherty, the general superior.

Despite its small enrollment, the school maintained its full senior program, Sister Michaela said, with 15 full-time and part-time teachers, five of them nuns. The largest classes -- English and religion -- had 20 students each. The smallest, advanced foreign languages, had only two or three.

"We promised that each girl could receive the full academic program," she said. "And they did."

"It was the best year since I've been here," said Kerry Kopta, the class speaker, who commuted to the school from Bowie. "I got to know everyone really well. We became close, very close to each other."

In 1984-85, the school had 455 girls in the four high school grades, 66 percent from Maryland, 31 percent from the District, and a few from Virginia. About 16 percent were black, a figure that rose to about 25 percent in the final class.

The announcement in October 1984 that the school would close prompted a massive protest by parents and teachers. Although a parent group said it could match American University's offer, the sisters said they could not break their contract with the university. The parents then sued to block the sale, but their claim was rejected by D.C. Superior Court Judge Geoffrey M. Alprin.

On Tuesday, a three-judge panel of the D.C. Court of Appeals heard arguments in an appeal of the case. Yesterday, Sister Anne Doherty said the transfer to American University is scheduled for mid-June, but would be delayed if the court does not rule by then or issues a ruling against the sisters.

Meanwhile, the new Immaculata, housed in an old Montgomery County elementary school, is trying to carry on the education and traditions of the old one, but without any nuns. This year it had the same academic program and even the same uniforms as the old Immaculata, said its principal, Rita Gleason.

Its 300 students in grades seven to 12 include 59 seniors, all but two of whom were juniors at the old Immaculata last year. Their graduation is scheduled for May 30.

"I have some very good friends out there and I visited them," Ingraham said. "It's a very nice school, but it's just not the same."

Michael Gardner, a member of the new school's board, said it is probably the prototype of Catholic schools in the future. "The nuns are depleted because of age and retirements," he said. "So in Catholic education the parents will have to get into the act. And they want to."