When the Archdiocese of Washington sold Immaculate Conception High School in downtown Washington early this year for $2.5 million and used the money to renovate and expand a high school at 10th and Lawrence streets NE, it followed a growing trend in Catholic education throughout the country. Schools pressed by high costs and low student populations are selling buildings located on valuable commercial property to help finance their educational endeavors.

Cathedral High School in Los Angeles will be sold next year and its students moved into three other Catholic high schools in the city next year. Cathedral, located in Los Angeles' Chinatown near Dodger Stadium and Griffith Park, sits on property more valuable than that of the other three schools, said an archdiocesan spokesman, who did not name a sale price.

Church leaders say several factors contribute to the budget pinch suffered by many schools. The primary reason the "financial need" is greater now, however, "is because we didn't have to pay faculty" in the past, said Msgr. John F. Meyers, president of the National Catholic Educational Association.

Teaching nuns, priests and brothers, who generally are paid little or no salary, have declined in number sharply since the 1960s, when there were three religious instructors for every lay teacher -- one who is not a member of a religious order. Now the ratio has reversed, with the result that "salaries have shot through the ceiling," said Bruno V. Manno, director of research and in-service programs for the NCEA.

The number of lay teachers increases by 2 percent every year, indicating that, unless the trend reverses, Catholic schools soon "will be to a great extent staffed by lay teachers and policy will be made by boards of education made up by laymen," Meyers said.

A combination of factors has led to the shrinking ranks of religious teachers. Fewer young men and women are entering religious orders, and those who do now have a wider variety of professions to enter in contrast to the traditional jobs of teaching and nursing to which women in particular were restricted in the past, say church officials.

The decline in the number of young women and men choosing to devote their lives to the church has brought another financial concern: the cost of caring for aging members of the orders with fewer young members making contributions. Half the women in U.S. religious orders are over 59 and half the men are over 52, an increase of 3.5 years in the median age for both groups during the period from 1976 to 1981, according to a survey published by the National Conference of Catholic Bishops.

This financial need led to the recent announcement that Immaculata Preparatory School, a successful and respected high school for girls, and Immaculata Dunblane, an elementary and junior high school, will be sold to American University in 1986 dditional enrollment declines in Catholic schools came when middle- and upper-income Catholic families joined the flight to the surburbs after World War II, leaving behind the inner-city parishes that were the traditional power base of the church's educational system. In the suburbs, they found fewer Catholic schools and high construction costs for building new ones.

The biggest numbers of Catholic school closings came in the late 1960s and early 1970s, with the number of schools dropping from 13,292 in the 1965-66 school year to 9,401 last year, when only 31 schools were closed. "We haven't closed many schools in the last few years. And the most recent closings relate to population shifts or population declines," Meyers said.

The post-war population movements brought other changes to the urban landscape, with retail businesses, white-collar professionals and industry moving into former residential areas. In some areas, Catholic schools with fewer pupils and higher operating costs, such as Immaculate Conception in Washington and Cathedral High in Los Angeles, found their property had spiralled in value.

The sale of Immaculate Conception, which had 237 girls in its student body at the time, was "part of a long-range plan" to upgrade Catholic education, said the Rev. Donald Essex, secretary for support services in the Washington Archdiocese. "Educators are saying that schools with under 400 students are not good schools, financially and academically" and that "we should close schools if they do not have the resources for such things as good math and science courses." Immaculate Conception's three-story building at 24th and K streets NW was built about 75 years ago, and the high school moved there in 1954. Since the school was founded at Eighth and Q streets NW in 1865, the two former residential neighborhoods have changed completely, and are now filled with office buildings, stores and restaurants.

Following church recommendations that small schools be amalgamated, archdiocesan officials decided to consolidate Immaculate Conception and St. Anthony's, a coeducational high school at 10th and Lawrence streets NE, Essex said. The proceeds of the downtown high school's sale were used to renovate St. Anthony's and add a new building, to make All Saints High School, which was dedicated Nov. 1, he said.

The story of Cathedral High in Los Angeles is similar. The school population has dropped from 700 in the mid-1960s to about 460 this year. The buildings are old, need substantial repair to meet the city's earthquake code, and are in a non-residential neighborhood, said Msgr. Jeremiah Murphy.

Regis High School in New York City, while not planning to sell despite increased costs, may occupy the most valuable property of any Catholic secondary school. It is located on a large chunk in the middle of an upper East Side block in Manhattan, an area that a New York real estate analyst described as a "fantastic neighborhood." He estimated that the property could be sold for $12 million to $14 million if there were no zoning or historic preservation limitations, community opposition or other restrictions, but added it would be unusual for property in that area to have no restrictions.

The boys' school, an academically elite institution with 495 students, was founded and endowed in 1914 by an anonymous benefactor who left a will specifying a number of conditions for the school's operation, said the Rev. Frederick O'Brien, the headmaster. One stipulation is that Regis cannot charge its students tuition.

While the "1914 dollar doesn't go as far now as it did then" and the number of Jesuit teachers has dropped to about one-third of the faculty over the last 25 years, Regis alumni are generous with their contributions, and "we don't consider our financial situation desperate," O'Brien said. If it did become desperate, "sale of the building might occur as a possible solution . . . but it would have to be studied by our lawyer" to determine whether the founder's will would allow that.

Other inner-city schools, however, struggle along in neighborhoods where middle-class families who migrated to the suburbs have been replaced by poor, often minority, residents. Parents cannot afford to pay as much tuition or contribute as much to fund drives as their predecessors, leaving school administrators to cope with fewer resources to cover higher costs.

"Particularly revealing" is St. Peter's High School in San Antonio, according to a report published by the National Center for Research in Total Catholic Education. Families who moved from the city to suburban neighborhoods were replaced by black and Hispanic poor, with the result that the median family income of St. Peter's parents is now $20,000, usually for a two-income household. The school needs money to attract and keep qualified teachers, renovate its deteriorating physical plant, and add business, computer and vocational courses.

" . . . The fiscal picture for many inner-city schools such as St. Peter's seems ominous," the report said. "Even though the demand for such schools is currently strong, they face a situation in which tuition increases cannot generate anywhere near the additional resources needed."