Between them, Bettie Benjamin and Nate Bush have 21 years of experience on the D.C. school board. Everywhere they go these days, they carry the curse of the incumbent: While their opponents bemoan the state of public education, they end up defending the schools, sometimes to the groans of their audiences.

But incumbency also carries advantages, and when Benjamin attracted three opponents in Ward 5 and Bush drew two challengers in Ward 7 this fall, the odds that either board member would be unseated dropped dramatically. The challengers agree their number makes them long shots, but in neither race has anyone been persuaded to drop out to allow head-to-head campaigns.

The result is generally quiet races in which the candidates agree on most educational issues, while challengers try to maneuver Benjamin and Bush into defensive and unrealistic positions. The incumbents fight back with a combination of pride in the system's progress and recognition that city schools are still plagued by low achievement and poor building conditions.

On Nov. 3, District voters will find one at-large school board seat and an initiative on school funding policy on the ballot. In addition, voters in Wards 1, 4, 5, 6 and 7 will elect ward representatives to the school board. While most of those wards' board members face only minimal challenges this year, the races in Wards 5 and 7 have the incumbents running hard, board members say.

"There are really only two races this year, in 5 and 7," said board member R. Calvin Lockridge (Ward 8), who is not up for reelection. "Both Benjamin and Bush could be in real trouble -- if the field narrowed. But unless the races are head-on, they should be okay."

While most of the city's educational and political establishment has lined up firmly behind the incumbent school board members, Benjamin and Bush have found cracks in their political armor.

D.C. Council member Harry Thomas (D-Ward 5) has endorsed challenger Angie King Corley against Benjamin. And both council member H.R. Crawford (D-Ward 7) and the Washington Teachers' Union have come out for Herbert Boyd in his race against Bush. Those endorsements, along with other support, have enabled challengers to raise nearly as much money as the incumbents, unlike the challengers in all other school elections this fall.

Benjamin, a lawyer who heads the board's rules committee, has emphasized a handful of achievements in speeches at sparsely attended community forums in Ward 5 in upper Northeast. She has specialized in early childhood issues and points to the system's prekindergarten classes, computer labs and tougher discipline policy as evidence of improvement.

"The schools are doing a good job," she said. "It's a common campaign tactic to detract from what the incumbent has done, but I know we do have problems."

Although she is late in filing a finance report that was due Oct. 10, Benjamin said yesterday that she has raised "close to $5,000" in her reelection bid. That is somewhat more than her opponents have raised for their posters, buttons and phone banks.

Samuel Robinson, a challenger who is a training specialist for the D.C. Department of Human Services, has collected $3,754 -- $1,000 of it from the Florida Avenue Baptist Church, where he is on the board of trustees. Corley, a guidance counselor at McKinley High School and a 30-year veteran of the school system, has raised $3,002, with fellow teachers accounting for many contributions. And Kathryn Pearson-West, a parent of two children at Bunker Hill Elementary, has raised $1,260, much of it in small gifts from local residents.

Robinson, 45, bills himself as "your back-to-basics candidate." An aggressive speaker who excites audiences, he has pledged to boost student achievement by increasing the number of required courses and focusing on low self-esteem among students.

"I am very disturbed when I see academic studies showing that our students associate achievement with acting white," he told a group at Webb Elementary in the Trinidad neighborhood Tuesday night.

Corley, 63, a soft-spoken critic of the school board's "general inaction," asks voters to recognize her experience in the schools. She said she would concentrate on lowering the system's 35 percent dropout rate and raising teacher salaries.

Council member Thomas said he endorsed Corley because "she worked on my campaign and you have to stick with your friends. We have big problems with drugs around schools and walls falling down and hot water systems failing inside schools. Benjamin has been there 12 years and we still have these problems. Somebody hasn't been doing their work."

West, a 30-year-old staff assistant at the D.C. Department of Housing and Community Development and president of the North Michigan Park Civic Association, has been active in parent and community groups working against drug abuse and teen-age pregnancy.

She said she decided to run because, "for my children to get into the best colleges, this has to be the best school system. As it stands right now, that's not the case."

The Ward 5 challengers argue that voters deserve a fresh face. Robinson says that should be a former youth counselor such as himself. West says the new face should be that of a parent of D.C. public school children, which she is. And Corley says it should be that of a school employe, as she is.

Benjamin, who has the support of the teachers union, responds that with a new superintendent to be chosen in coming months, now is the time for experience.

In Ward 7, which occupies the eastern corner of the District, the race between Bush and Boyd is by all accounts the tightest in the city. The third candidate, James Miles, a lawyer, has raised no campaign money and is hard-pressed to compete with the heavy poster and brochure drives of his opponents.

Bush, a 38-year-old lawyer who heads the board's Finance Committee, easily defeated two candidates in 1983. This time, he is running hard for a three-year term. He has raised $9,722, much of it in small donations from school system employes and ward residents.

"Eight years ago, when I was elected, the Board of Education was a point of profound embarrassment and disappointment to our city," he said. "We have made the board more professional, stopped the bickering, improved student achievement and improved relations between the board and teachers."

Bush paints a picture of a peaceful school system operating with full funding, smaller classes and improved discipline. But Boyd, a 35-year-old social worker in the public schools and son of a prominent former principal, says there may be too calm an atmosphere on the current board.

"We're seeing a lot of rubber-stamping," he said. "We have to take action to save many youngsters who will more than likely be destructive forces in the community."

The challenger, who worked on Bush's campaign four years ago, said he and Bush agree on most issues. "But it's one thing to agree and quite another to then work on those issues," Boyd said.

Like the other incumbents, Bush stresses the need for experienced hands at a time when the school system is moving to repair long-neglected buildings, pick a successor to Superintendent Floretta D. McKenzie and reduce class sizes.

The tightness of the race was reflected in the teachers union decision. While a screening committee selected Bush, the union membership overrode that vote and picked Boyd.

"The members felt Bush has not carried his weight on the board," said union President William Simons.

Council member Crawford said he chose Boyd because "once Bush wins the election, you don't see him again. Teen-age pregnancy is the highest in the city in my ward and something's got to be done."

Miles' campaign stresses improved academics for children who he says have a shallow understanding of history and a deep fear of science.