Barbara Lett Simmons, running for an unprecedented fourth-term as an at-large D.C. school board member, was defeated yesterday by parent activist Phyllis Young, while Ward 8 incumbent R. Calvin Lockridge won a close race, according to final but unofficial election returns.

Young, a branch chief at the U.S. Department of Transportation, who had singled out Simmons in a race for one of the two available at-large seats, beat the longtime incumbent by 3,000 votes, according to the returns.

In other races, David Eaton, the other at-large incumbent, led a field of five at-large candidates, winning with more than 26,000 votes.

R. David Hall, the Ward 2 incumbent, trounced his sole opponent, Charles Briody, by 3,988 votes to 647, according to the returns.

Ward 3 incumbent Wanda Washburn ran unopposed.

Simmons, a feisty 12-year veteran member, was the center of several controversies during her long career on the board. As a vocal critic of popular school superintendents, including the current one, Floretta D. McKenzie, Simmons built a reputation as a sharp-tongued, combative member who was a staunch advocate of inner-city schoolchildren and supporter of vocational education.

Young, 44, who was making her second run for an at-large board seat, said last night she was so excited that "I can't even think right now. I see it as a victory for the children. I think that people will stand up for positive role models for children. And they recognize the importance of elected officials."

In 1980, Young cofounded Parents United, a coalition that lobbies the city government and Capitol Hill for full funding of school board budgets. She was supported in her election bid by City Council member Nadine Winter (D-Ward 6) and Ward 6 school board member Bob Boyd.

"I believe a lot of it had to do with teamwork among groups of people who would not normally be political allies, but all of whom want to see improvement in the schools. People put down differences for the benefit of the kids," said Young, who has two daughters, ages 24 and 15.

Simmons, in an interview at what was to have been a victory party at a chemical warehouse in Northeast, said she was not yet ready to concede defeat. "I don't know about the integrity of the vote tabulating procedure," she said. "It certainly has been in question in the past."

Appearing a bit shaken but still confident, she added, "I will always be there for young people, the distressed, the oppressed and the poor. That capacity to stand up for others and to care has not been replaced on the board by this election . I have a superior capacity to care and to do . . . . I won't look back with regrets . I have no intention of looking back."

In Ward 8, Lockridge, who was running for a third term, beat his closest competitor, Absalom Jordan, by 30 votes. Jordan, who was making his second try for the school board, had led Lockridge in early returns.

City election officials reported that voting was light yesterday, as is usual for school board races, which have always been held in odd-numbered years. Some observers had predicted, however, that a referendum on parts of the city's rent control law could bring more voters to the city's 137 polling places.

The school board contest is a nonpartisan election open to all of the city's 263,000 voters. The two candidates with the highest number of votes in the at-large race are declared the winners.

The low turnout reflected the school board candidates' campaigns, which were marked by sparsely attended forums and a general lack of interest around the city.

Candidates courted city residents by telephone, cajoled them via loudspeaker-equipped motorcades, knocked on their front doors and sought them out at bus stops, beauty shops and street corners. The candidates raised more than $15,000 to finance their campaigns.

In the at-large race, observers said early on that the real contest was between Simmons and Young. Young, perhaps the best known of the challengers, ran for an at-large seat in 1981 and came fourth. This time, she opened her campaign by announcing she was "running for the seat held by Barbara Lett Simmons."

In Ward 8, the city's poorest area, Lockridge faced five challengers until Sunday when one dropped out. Lockridge reveled in the large field, saying, "It's to my advantage to have a lot of challengers. They divide the anti-Lockridge vote." His challengers agreed that they would stand a better chance with fewer candidates and tried to thwart Lockridge's strategy by joining forces with some of those who had lost to Lockridge in 1981.

Other candidates in the race were Virginia Howard, a University of the District of Columbia professor and trustee, who placed third with 725 votes; Jacqueline Shillings, 52, a registered nurse; Frank Sewell, 32, a former apartment manager who is unemployed; Lin Covington, 50, a D.C. mathematics teacher, and William Lewis, 39, a letter carrier, who dropped out of the race on Sunday, too late for his name to be deleted from the ballot.

In preparation for the school board races that will occur with other citywide elections beginning in 1988, the two at-large members, Eaton and Young, and the Ward 3 member, Washburn, will serve three-year terms, while Hall and Lockridge from Wards 2 and 8 will serve five-year terms. School board members previously served four-year terms.