A lawsuit to gain control of Immaculata Preparatory School, a prestigious girls' Catholic school in Northwest Washington, was dismissed yesterday by a D.C. Superior Court judge.

A group of parents and graduates filed the class action suit after plans to sell the 79-year-old school were announced last fall, claiming that the Catholic nuns who arranged the sale defrauded students and contributors for their own "personal enrichment" and knew the school was going to close when they admitted new students last spring.

Supporters of the suit against Immaculata said yesterday they plan to appeal the decision by D.C. Superior Court Judge Geoffrey M. Alprin.

Sister Michaela Galvin, principal of Immaculata, said of the decision to dismiss the suit, "All I can say is that we are just very happy,"

The suit argued that the sisters who ran the school concealed the sale of the 8.2-acre site and had "no intention" of providing a four-year education to students who are now freshman and sophomores. On Oct. 2, 1984, the Sisters of Providence of Saint Mary-of-the-Woods announced that they planned to sell the property at Wisconsin and Nebraska avenues NW to American University for $7.6 million.

The suit asked that title to the property be turned over to a new group of trustees who would continue to use it to provide a Catholic education for young girls.

At the time the suit was filed, officials of the order, based in Indiana, said the school was being sold to provide for the care of the large number of aging nuns in their ranks.

Since the sale, parents announced they would reopen the school this summer in Rockville under a new name, Immaculata College High School.