Supporters of Immaculata Preparatory School in Northwest Washington filed suit yesterday to wrest control of the institution from the order of Catholic nuns that plans to sell it, claiming the nuns have defrauded students and contributors for their own "personal enrichment."

The class action suit, filed by a group of parents and graduates in D.C. Superior Court, contends that the nuns knew the school was going to be closed when they admitted new students last spring, but that they withheld that information from parents.

The suit asks that title to the 8.2-acre property be turned over to new trustees who would continue to use it to provide a Catholic education for girls, as originally intended. The sisters announced Oct. 2 that they planned to sell the campus to American University for $7.6 million in 1986. The property at Wisconsin and Nebraska avenues NW is occupied by Immaculata Prep, a high school, and Immaculata Dunblane, a grammar school and junior high.

After the suit was filed, Sister Ann Doherty, general superior of the order, said in an interview, "We have not been guilty of any fraud . . . . The money will not be used for the personal enrichment of our members, but to feed, house, clothe, and provide health care for our elderly sisters, many of whom taught at Immaculata for many years and took very meagre stipends."

The order, whose headquarters is in Indiana, has a national membership of 955. More than a third of them are over 70 years old, Doherty said, and only about half are able to work full time.

"It's always painful when rights are in conflict," she added. "But we should not be condemned for choosing the rights of the aged and infirm who have given their lives for the education of young people."

The group that is trying to forestall the sale of the 79-year-old school, which is called Save Immaculata/Dunblane, also has been holding discussions with officials of the Washington archdiocese, including Archbishop James A. Hickey, who earlier had given the nuns permission for the sale.

Yesterday, Richard A. Graham, the group's lawyer, said the discussions had been "positive." The group had considered including the archdiocese as a defendant in the lawsuit, but Graham said it had decided to sue only the nuns, who "absolutely won't talk to us."

According to the suit, the planned sale would destroy the parents' ability to fulfill a church mandate that they provide their children with a Catholic education.

Parents have a right to benefits from the school, the suit contends, because of financial contributions they have made over the years, including $300,000 from the fathers' club, $80,000 from the mothers' club and between $5,000 and $6,000 from each graduating class of the preparatory school.

The suit accuses the nuns of "intentional omission, misrepresentation, concealment, lulling and false promises," claiming they had "no intention" of providing a complete, four-year education to students who are now freshmen and sophomores at the preparatory school.