Charles Woodward High School in Rockville and Walter Johnson High in Bethesda will be run as joint campuses for one year before the two schools are merged in the fall of 1988, the Montgomery County Board of Education said last night.

The board voted last week to close Woodward and consolidate it with Walter Johnson in an effort to deal with underenrollment at both schools. Enrollment at each school was expected to fall below 800 students by 1990.

Woodward's ninth graders will begin attending Walter Johnson next year.

Board members indicated they believed that a joint-campus operation would best facilitate the consolidation, but Bob Bernstein, president of the Woodward PTA, opposed the move, saying in a letter to the board that it would create many scheduling problems for Woodward students.

He said it would be difficult for students to juggle classes at the two campuses, with Woodward students bused a half mile to Walter Johnson for some classes. "The proposed combined campus operation would be highly inefficient and detrimental," Bernstein's letter said.

Bernstein also said he would recommend that the Woodward PTA board appeal to the state Board of Education the decision to close Woodward.

The school board also finished last night approving portions of a $90 million capital improvement program that calls for two new elementary schools and a new high school in the fast-growing northwestern part of the county. The entire capital budget request now goes to County Executive Charles Gilchrist who can modify it before passing it on to the County Council for final review.

Last year, the council approved a $23 million capital budget for the schools.

School officials say the new schools in the Germantown-Gaithersburg area are desperately needed to relieve overcrowding. Northwestern Montgomery County is in the midst of a major real estate and development boom that is attracting many young families with school-age children.