Washington's Catholic Archdiocese yesterday announced the merger of two high schools for girls in central Washington and said there are plans to move Good Counsel High, a school for boys in Wheaton, to Gaithersburg within a decade and make it coeducational.

The changes, announced by Archbishop James A. Hickey, are part of a new master plan for the area's Catholic high schools, which church officials said will cost between $25 million and $30 million to implement.

In addition, Hickey said all students in area Catholic schools, including the approximately 25 percent who are non-Catholic, will be required to attend classes in Catholic doctrine and "to be part of all religious exercises with the exception of receiving the Sacraments by non-Catholics."

"Our schools are Catholic, religious schools," Hickey declared. "They are not to be considered simply an alternative to public education."

At present individual schools have different policies about religious instruction and observances by non-Catholics.

Hickey said the plans for closing some school buildings and constructing new ones were necessary because of falling enrollments in many Catholic schools in the District and its close-in suburbs, where the population is dropping, while the demand for such schools is rising in fast-growing suburban areas farther out.

According to the new plan:

*St. Patrick's Academy, at 924 G St. NW, will merge next fall with St. Cecilia's Academy, 601 East Capitol St. The new school, to be called Holy Spirit High School, will be located on St. Cecilia's property, which will be purchased by the Archdiocese for $1.5 million from the Holy Cross Sisters. The two schools now have a total enrollment of about 325.

*Good Counsel High, at 11601 Georgia Ave., Wheaton, will be moved to Gaithersburg, probably near the Shady Grove Metro station. The new school will accommodate 1,000 to 1,200 students and cost $10 million to $12 million to construct.

Hickey rejected a request by the Xavierian Brothers, who own the school, that it become coed now, saying that would harm several all-girls schools nearby. He said negotiations are under way with the order about the financing and ownership of the new Gaithersburg facility.

*St. Mary's Ryken, a coed high school in Leonardtown in St. Mary's County, will construct a new building for up to 1,200 students near the Charles County line. The new school, costing $10 million to $12 million, will be close to Rte. 5 and convenient to St. Charles City.

*An addition to house seventh and eighth grades will be constructed at St. Vincent Pallotti High School in Laurel, a rapidly growing area. Catholic high schools in Montgomery County will make arrangements to admit students who score as low as the bottom 30 percent on nationally standardized tests instead of restricting admissions to those above the national norms.

*Bus service will be established to take students to Catholic schools from areas where no public transportation is available. A sliding scale of fees will be based on family income.

Hickey's announcement yesterday was based on the recommendations of a 20-member archdiocesan high school study committee that he appointed in April 1983, but Hickey made major changes in the committee proposals, which were presented to him last June.

The Washington Archdiocese, which includes the District and five counties in Maryland, has 24 Catholic high schools enrolling about 11,500 students. Twenty are owned by religious orders and four by the archdiocese. In addition, about 24,000 students attend more than 100 Catholic elementary schools in the area.

Since 1970, Catholic high school enrollment has dropped by about 8 percent, John Convey, a Catholic University education professor who served as project director, said projections indicate a much steeper decline over the next decade.

Convey said the scheduled closing of Immaculata Preparatory School in 1986 did not affect the plans announced yesterday.

Although the closing, which has aroused strong protests, was first disclosed last month, Hickey gave approval in June for the Sisters of Providence, the order that owns the school at Tenley Circle NW, to sell it to American University.

Yesterday Convey and Bishop Thomas W. Lyons, chairman of the high school study panel, said Hickey had turned down a request by the Holy Cross Sisters last summer for permission to sell St. Cecilia's, telling them to wait until he had completed action on the master plan.