IN FAIRFAX COUNTY, one seat is up on the board of supervisors -- to replace a Democrat who resigned in the Mount Vernon district. The race is a hot one because a Republican victory would 1) give the GOP its first board majority in this century and 2) upset the current useful balance of a Democratic board with a Republican chairman. Gerald Hyland, Democrat, is a first-rate candidate who has served ably on local zoning, public safety and human rights boards. T. Farrell Egge, Republican, has a firm grip on Ronald Reagan's national coattails and a record devoid of any claim to local service. It's tight, and the danger is that a third-candidate spoiler, independent Gerald Fill, will divert just enough Hyland votes to throw the race Mr. Egge's way.

Also on the Fairfax ballot is a $75 million school bond referendum. In a county proud of its support for public schools, it's in trouble. Crowding is severe in the county's growing western end. The school board could have eased some of it by adjusting school boundaries, but it caved to neighborhood pressures. That left it with little choice but to load up the referendum package with questionable renovation projects in the eastern end in order to lock in votes for priority construction in the west. As a result, important citizen groups that generally support school bonds and whose support is needed to pass them are either opposed or neutral. Parents at Fort Hunt High School, an underpopulated eastern Fairfax school recommended for closing by a citizens' task force, are baldly conditioning their support on a board pledge to keep their school open.

The unrelated fuss over the pay of retiring superintendent William J. Burkholder may have influenced Fairfax voters' attitude toward school issues this fall. But the deeper difficulty is that the school board has allowed important boundary, school-closing and financing questions to be drawn excessively into the political thicket. This school bond referendum is not well enough drafted. It needs to be amended and resubmitted next year by a school board that, in the meantime, demonstrates a greater degree of political independence.

The two other Fairfax bond issues, for neighborhood improvements and storm drainage, are your basic suburban necessities.