Immaculata Preparatory School and its sister primary school, mainstays of Catholic girls' education here for 80 years, will be unified under the roof of a former Rockville elementary school building as part of a $70,000-a-year agreement with Montgomery County, hundreds of supporters were told yesterday.

Parents, students and teachers who met to take a look at the new home -- Montrose Elementary School, off Academy Way -- got the first confirmation that Immaculata Prep, once a four-year high school, and Immaculata Dunblane, a separate school for grades five through eight, will become Immaculata College-High School, for grades seven through 12.

"We offer you the opportunity to maintain your alma mater," John Miller, a leader in a parents organization formed to find a new school site, told a standing-room-only crowd in the Montrose lunchroom. "You can be in attendance at the rebirth of Immaculata."

The future of the two schools, now at Tenley Circle, had been unknown since the Catholic nuns who operate the schools decided in October to sell the 8.2-acre campus to American University.

Representatives of the Sisters of Providence said they needed the $7.6 million from the sale to care for the order's elderly. Since then, a group of parents known as Save Immaculata-Dunblane Inc. has filed a class-action suit against the nuns. The suit charges that the nuns defrauded students and contributors by selling the school for the nuns' "personal enrichment."

While pursuing the legal action, the parents group began looking for a site for the school. The group signed a contract Thursday with the International Center for Public Safety Training, a private organization that has leased Montrose Elementary from the county and operated out of the building for the past three years.

According to the contract, the Immaculata parents group will assume the remaining seven years on the lease agreement between the training center and the county. The parents group will have the option to lease for an additional 10 years, group leader James Spelman said.

The lease agreement, which puts the Immaculata group in charge as of April 1, values the 34,000-square-foot building at $2.05 a square foot, an annual price tag of $70,000, Miller said.

The present school complex, about a mile from the Twinbrook Metro station, has 28 classrooms and a lunchroom that doubles as a gym. School supporters were told yesterday that 300 students would be needed to make the school a feasible operation the first year, which is expected to begin in September. Attendance eventually should meet the total of students now attending both Immaculata schools, about 600.

Miller said the school would be operated by a board of directors acting under the auspices of the Catholic Archdiocese of Washington. About 20 of the school's 38-member lay faculty have expressed an interest in continuing to teach at the school following its relocation, Spelman said. Whether any nuns will teach, "we do not know," he said.

"It's not maverick at all," Miller said of the lay nature of the faculty. "In fact, it's the way of the future. The Vatican Council emphasized the need for lay people to get more involved in the church, and, as religious orders get older, more and more lay people will have to get involved in maintaining its education."

Annual tuition at the new school is expected to be $350 higher than the $2,650 in tuition and fees students now pay at the Immaculata schools, parents said. Classes for seniors and eighth-graders -- the two levels whose students have been given the option of finishing their education at the old facilities -- will cost the same as tuition at the new school, $3,000.

That price tag was shrugged off by most members of the audience yesterday, who said they believed that no cost was too high to ensure the high academics and the Catholic orientation of the schools.