A headline in yesterday's Washington Post incorrectly stated that Mary Margaret Whipple of Arlington will oppose a school board chairman in the county's November elec- tions. Whipple is seeking a seat on the Arlington County Board currently held by that board's chairman, Stephen H. Detwiler.

Former Arlington school board chairman Mary Margaret Whipple announced yesterday that she will oppose County Board Chairman Stephen H. Detwiler in the November elections, but left unanswered the question of whether she will run as a Democrat or independent.

Whipple, 41, who failed to win a seat on the County Board in the 1979 elections, said she would leave the decision on how she runs to members of the Arlington Democratic Party. They will vote at a May 10 mass meeting whether to nominate her or endorse her as an independent.

Detwiler, 38, announced in December that he would seek a second four-year term on the board as a Republican-endorsed independent. He will seek the GOP's backing at a May 11 canvass of party members. Detwiler and GOP-backed independents Dorothy T. Grotos and Walter L. Frankland, whose terms expire next year, constitute the current majority on the five-member County Board.

As independents Whipple and Detwiler could seek the active support of the large number of federal employes in Arlington who are barred by the Hatch Act from participating in partisan politics.

If Whipple runs as an independent as she did in 1979, she likely would be assured the support of Arlingtonians for a Better County (ABC), a self-described nonpartisan organization with a large federal-employe membership and a history of endorsing Democrats who run as independents. The group will vote May 26 on whether to support her.

The Arlington Democratic Committee has gone on record in favor of nominating a candidate this year, and committee chairperson Sharon Davis said yesterday it was "highly unlikely that there will be enough support to go the endorsement route."

"Ideally," Whipple said, "I'd like to run as an independent to the first of November and then as a Democrat on election day (Nov. 2). I can't help but think this would be a good year to run as a Democrat. But one of the chief disadvantages of running as a Democrat is losing the active, full participation of Hatched federal employes."

Flanked at her news conference by County Board members John G. Milliken, who won as a Democrat in 1980, and Ellen M. Bozman, who was elected as a Democrat-backed independent last year, Whipple stressed her desire to overturn the board's current majority.

"There is a clear choice presented to the voters this year that goes beyond the single board seat that is up," said Whipple. "It goes to the question of which majority do you want. Do you want a new majority of Bozman, Milliken and myself, or do you want to perpetuate the drift of the current majority of Frankland, Grotos and Detwiler?"