WITH ITS high school enrollments forecast to keep declining slightly through this decade, the Montgomery County School Board proposes to build two new high schools. County Executive Charles W. Gilchrist has now postponed one of those new high schools until the 1990s. But school board members argue vigorously that in the areas of high growth north and west of Gaithersburg, it's necessary to build immediately.

Mr. Gilchrist responds that this growth can be handled by shifting enrollment boundaries, and that building both of the new high schools would threaten the county's bond rating. He would go ahead with the Quince Orchard High School, but not with Watkins Mill. The battle is on.

The school board is asking too much. But it's a good board -- the best that the county has had in some years -- and its case deserves attention. While the present buildings can accommodate the county's high school enrollments in the arithmetical sense, that would impose costs of another kind. One member, Blair Ewing, says, "It will mean constant shifting of students, constant boundary changes, overcrowded classrooms upcounty, a very considerable increase in long-distance busing of students, a lack of stability and continuity in children's education." The board's president, Robert E. Schoenberg, adds that the increased population is surely coming and the board doesn't like to keep moving kids from school to school as enrollments rise. That's an admirable goal. The question is the amount of money that even a wealthy county can devote to it. The county's current six-year school construction program would cost $93 million. The school board wants to increase it for the years 1986-91 to a swinging $248 million. Mr. Gilchrist has cut it down to $162 million, but tht's still a 75 percent increase over the present schedule -- not a small acceleration.

The bond rating is getting too much attention in this quarrel. There's another sanction that's more important. From the middle 1970s to 1982, the Montgomery County schools had a very rocky time under a succession of bitterly divided and frequently misguided boards. A taxpayers' revolt contributed heavily to the results of those years' school board elections, and the impact on the school system was substantial. That time has passed, fortunately, and the present board was elected by good friends of the schools -- led by PTAs and teachers' organizations. It would not be smart of the schools' friends to invite a repetition of the past decade's experience.

Montgomery County has large resources and supports a fine school system. But it will serve the schools' interests to set a limit -- as Mr. Gilchrist is doing -- on the speed of the expansion that is now under way.