Carrying candles and reciting prayers, about 250 supporters of Immaculata Preparatory School marched outside the Hyattsville home of Archbishop James A. Hickey last night, seeking his help to prevent the planned closing of the Catholic, all-girls school.

They received little encouragement that the school at Tenley Circle NW might be saved.

Hickey, who last summer approved the sale of the school to American University for $7.6 million, was at a church several miles away during the somber, hour-long demonstration, a spokesman said. One of his top aides, Msgr. John F. Donoghue, vicar general of the Washington Archdiocese, read a statement by Hickey, saying the archbishop was "saddened" by the closing and had appointed a task force of six church members to discuss it with parents.

But the statement said the purpose of the task force was "to explore alternative plans for students unable to continue at Immaculata," which is owned by the Sisters of Providence, an order of Catholic nuns. Donoghue, who will head the task force, told reporters: "I think the sale of the school is a foregone fact. It's the only way to provide care for the elderly sisters . . . I think they got the best deal they could."

The 8.2 acre campus, occupied by Immaculata Prep, a high school, and Immaculata Dunblane, a grammar and junior high school, is at Wisconsin and Nebraska avenues NW, a block from a new Metro subway station and just under a mile from American University. The schools were established at the site in 1905 and together enroll about 560 students, all girls.

An announcement last week by the Sisters of Providence, whose headquarters is in St. Mary-of-the-Woods, Ind., said the schools will close in June 1986 with settlement and transfer of the property taking place shortly afterwards.

After she heard Hickey's statement, Mary Theresa Landers, vice president of the Immaculata Prep Mothers Club, said she was "happy there will be dialogue" with church officials, but added, "We pray that Immaculata will be saved on its present site."

Others in the march, composed mostly of Immaculata parents and students, were less temperate.

"They sold it behind our backs," said Jane Armstrong, a parent from Northwest Washington. "If we had been given a chance to help the nuns we certainly would have, but not at the expense of closing the school.

"The church tells us to have children," Armstrong continued. "It tells us to give them a Christian education, and then it lets the Catholic schools disappear."

"How can the archbishop give $600,000 to the McKenna House for homeless men a church-sponsored shelter in Northwest Washington and have no money to help save Immaculata for the education of Catholic women?" said Susan Grayson, of Kensington. "We are his flock."

Last week, the Sisters of Providence said they had to sell the school, even though its enrollment and programs are flourishing, to provide for the retirement and medical care of aging members, more than a third of whom are over 70 years old.

"The mounting cost of care for their elderly . . . must be met," Hickey declared in his statement. "Vocations to the religious life and a sustained willingness for contributions to the needs of the religious orders themselves are facts that we must all face realistically."

Despite the discouraging response from the archbishop, the demonstrators continued to pray.

Just before the group dispersed, the marchers gathered by the steps of the Pastoral Center, which serves as the headquarters of the archdiocese as well as Hickey's residence. "Sing out like you never sang before," one parent urged them. And the girls sang the hymn "Our Lady of Providence."