The Arlington County Board's Democratic majority yesterday appointed Gail H. Nuckols to the school board, the first of three appointments the Democrats will make in the next 13 months that will give Democratic appointees a majority on the now all-Republican school board.

The County Board's two Republican members, who abstained in the Nuckols vote, asked for the reappointment of incumbent school board member Michael E. Brunner, a Republican appointee who is running for the County Board.

Nuckols has long been active in school affairs and is a member of Arlingtonians for a Better County, a self-described non-partisan civic group with a history of endorsing Democrats who run as independents for the County Board.

She will succeed Brunner, whose term expires June 30.

Brunner's seat is the first on the five-member school board to come up for reappointment since Democrats won control of the five-member County Board last fall with the election of Democrat Mary Margaret Whipple, a former school board chairman who nominated Nuckols yesterday.

Next June, the seats of school board chairman Evelyn Reid Syphax and member Claude M. Hilton also are up.

Board member Walter L. Frankland endorsed a motion by board member Dorothy T. Grotos to reappoint Brunner yesterday. Whipple, Democratic board member John G. Milliken and board chairman Ellen M. Bozman, a Democrat-backed independent, voted for Nuckols.

Nuckols, 41, is the PTA president at Yorktown High School. She has been a member of a special commission on the county's secondary schools, chairman of the six-year planning committee on school wide goals, a member of the advisory council on instruction and numerous other school organizations.

In other action the board deferred until June 4 a decision on a proposal to develop 12 clustered townhouses on a two-acre tract surrounding the Glebe House at 4527 N. 17th St. The board's decision was made at the request of dozens of citizens during a three-hour-long hearing.

The delay is intended to give nearby residents an opportunity to meet again with property owner Frank Ball Jr., and Preston S. Caruthers, who wants to build the townhouses, to negotiate possible changes in the proposal.

The citizens, according to Waverly Hills Civic Association president David Martin, want a chance to raise funds, variously estimated to be as high as $700,000, to buy and maintain the property.

Failing that, they hope to persuade Caruthers to redesign the townhouse cluster and to reduce the number of planned units from 12 to eight.

The board approved a 17-unit clustered townhouse plan on a five-acre lot in the 2600 block of N. Marcey Road., near Potomac Overlook Regional Park. A majority of the nearly 40 persons who attended that hearing appeared to favor the plan, which the board scaled down from the developer's 19-unit request.