WASHINGTON (AP) _ Education Secretary William J. Bennett today urged voters to beware of presidential candidates spouting empty rhetoric about school reform, waxing nostalgic about the good old days or dangling the promise of billion-dollar budget increases.

Bennett offered his personal guide to handicapping the candidates on education in a back-to-school speech prepared for delivery at a National Press Club luncheon.

Bennett suggested that voters judge Democrats on whether they are ready to break ranks with the National Education Association on merit pay, and Republicans on whether they are committed to improving the schooling of disadvantaged children.

The address came on a day when many of the 45 million U.S. school children were beginning the new school year, and three days before nine presidential contenders gather in Chapel Hill, N.C., to debate education issues.

There will be separate Democratic and GOP debates on education at the University of North Carolina on Friday. All seven Democrats and two Republicans - Rep. Jack Kemp, R-N.Y., and former Delaware Gov. Pete du Pont - have accepted invitations to the forum.

Bennett said education ''is arguably the number one domestic concern of the American people.''

Bennett, a Democrat-turned-Republican, said, ''too often in our political discourse, education has been used as an opportunity for candidates simply to market mawkishness.''

''They say that we ought to invest in our future and, most importantly, in our children's future; they proclaim that teachers are good and important people,'' he said.

''All these things are true,'' said Bennett, ''but truisms ... do not constitute serious planks in presidential platforms.''

''We need more than platitudes and more than promises of more money and deep concern,'' he said.

Bennett said he tried to steer clear of political partisanship in his checklist of questions for the candidates. But he attacked the l.86-million-member teachers' union, the National Education Association, which he called ''the most mis-named organization in America.''

NEA President Mary Hatwood Futrell said in response that ''Bennett's real gripe about the NEA is that we have stood between him and his efforts to undermine the federal role in education.''

Futrell said in a statement that ''for Secretary Bennett what really matters are the two B's - Bill Bennett - not the three R's.'' She said he was ''exploiting education to position himself for high office - the vice presidency or even higher.''

Bennett said the candidates should be asked:

-''What will they talk about besides spending?''

-''Will they withstand union pressure, particularly from the NEA?''

-''Should teachers and principals who do better than others be paid better?''

-''What will you do to ensure that the education reform movement reaches the disadvantaged?''

Too often in the past candidates have been ''gushing with sentimentality, gushing all over our children and all over the rest of us,'' he said. ''And like all good suitors, these candidates often have sought to authenticate their sentiments with gifts of fancy trinkets and baubles - in this case, billion dollar budget increases.''

Bennett, who joined the Republican party last year after a spell as the only Democrat in President Reagan's Cabinet, said, ''When it comes to education, Republicans sometimes speak as if nothing is more important than to save money.''

They ''lament the mistakes of the '60s and ... yearn for the good old days,'' said Bennett.

''In the coming campaign, Republicans must do better than look at education through the green eye-shades of the accountant,'' he said.

Bennett said lack of accountability is the biggest obstacle to improving U.S. schools. The country has stiffer and more immediate penalties for serving rotten hamburger in a restaurant ''than for furnishing a thousand schoolchildren with a rotten education.''

If the candidates want to promise more federal spending, they should add accountability measures to their plans and be sure to target the funds on such measures as merit pay, magnet schools and dropout prevention, he said.

Among the Democratic presidential contenders, Sen. Paul Simon of Illinois supports boosting federal aid for education and he has been a sharp critic of Bennett's efforts to cut the Education Department's budget.

Former Arizona Gov. Bruce Babbitt wants the federal government to assume the states' $20 billion share of Medicaid, provided they spend the entire windfall on public education.

Jesse Jackson has said he would raise the federal education budget by at least $5 billion, boosting it to $25 billion.

Rep. Richard Gephardt of Missouri would steer bonus grants to states that improve test scores and reduce dropout rates.

Sen. Joseph Biden of Delaware has proposed doubling the Head Start program for disadvantaged children and lengthening the school year by 30 to 40 days.

Massachusetts Gov. Michael Dukakis would increase federal aid to schools.

Sen. Albert Gore Jr. of Tennessee would focus national attention on the importance of excellence in education and opportunities for training.

On the Republican side, Vice President George Bush has proposed new tax breaks to encourage parents to save for their children's college education and has promised more aid for the disadvantaged through Head Start and remedial programs.

Du Pont champions vouchers for the disadvantaged to buy remedial instruction at public or private schools.

Alexander M. Haig Jr., the former secretary of state, supports tuition tax credits for private education and tougher standards for teachers and students.

Kemp supports tuition tax credits, magnet schools, the teaching of foreign languages as early as kindergarten and vocational training.