The Montgomery County Board of Education voted last night to close Cabin John Junior High School and merge it with Herbert Hoover Junior High in September 1987.

School officials said the consolidation is necessary because of declining enrollments at the two Potomac schools for students in grades seven through nine.

"The fundamental question is not will Cabin John be closed but when," said board member Jeremiah Floyd.

The board rejected a separate proposal to operate Hoover and Churchill High School as a joint campus ninth-through-12th-grade school starting next year and continuing through 1990.

Echoing the concerns of most of her fellow board members, Sharon DiFonzo called the joint campus plan a "bad dream" riddled with administrative and scheduling problems.

She said she also was concerned that students would have had to walk a quarter mile between the two schools in bad weather.

The plan, however, had the backing of both the Churchill and Cabin John PTAs.

"I'm disappointed," said Susan Ellenberg, co-president of the Cabin John PTA. "I think it would have been an exciting thing to try, but I appreciate their concerns." Under the joint campus plan, Cabin John would have been a seventh- and eighth-grade intermediate school until 1989 or 1990, when Churchill would be converted into a ninth- through 12th-grade school. At that point, Cabin John would have been closed and its seventh and eighth grade students moved to Hoover.

Churchill is one of the county's few remaining 10th through 12th grade high schools. Most of the other high schools have been converted into four-grade high schools.

The Hoover PTA favored the plan to merge the two junior high schools. "We all think it's in the best interest of the students and the community," said Stephen Loeb, co-president of the Hoover PTA. Enrollments at Hoover and Cabin John were expected to decline through 1995 to below 600 at each school.

Hoover, with room for 1,031 students, has just over 600 students this year. Cabin John has room for 995 students but has enrolled fewer than 500 students this year.

Although Churchill High School is at 100 percent capacity this year and does not face as serious an enrollment decline, it, too, is expected to lose students slowly over the next nine years.

School officials said the merger of the two junior high schools will increase class offerings and strengthen the academic program.

The board did not indicate when it will transform Churchill into a school for grades nine through 12, but said it will reevaluate the situation every year.