President Reagan's education secretary, William J. Bennett, today ventured into a classroom here to teach black third-graders about the meaning of Martin Luther King Jr.'s life.

As he later noted, the well-drilled students at John Hope Elementary School were way ahead of him.

Why are all these cameras here and what makes King so important? he asked, after a bit of getting acquainted.

"He changed the laws to make things better for black people!" answered Rhonda Smith, 8.

What was King's dream?

"That black people don't get judged by the color of their skin," another child answered.

"How should we judge a person? . . . How should we decide if we like somebody?" Bennett asked.

"By the content of their character," came the answer, a direct quote from King's "I Have a Dream" speech.

"Boys and girls," Bennett said, "I wish everybody knew this lesson as well as you do."

Bennett's appearance was part of a 10-day King celebration here marking the first national observance of the slain civil rights leader's birthday. Although the national holiday will be observed Monday in many states, his birthday is Wednesday.

Today was "Youth and Student Day," and its theme was the importance of keeping King's legacy alive for generations born after his assassination in 1968. Bennett was invited by King's widow, Coretta Scott King, head of the King Center for Nonviolent Social Change around the corner from the school.

She said she had been impressed with Bennett's knowledge of her husband's teachings when she met with him last summer in Washington to give him some textbooks about King coming into use in schools.

Bennett, who has held classroom teach-ins nationwide, said that as a teacher at the University of Southern Mississippi in 1968, he had joined several other faculty members and students after King's assassination in a seminar on King's achievements. "As a result," he said, "a threatening sign was placed on my office door and several people made anonymous threats."

The children at John Hope, most of them from low-income families and many from single-parent families, were eager. They had known since last Tuesday that Bennett was coming, their teacher said, and had their King textbooks at the ready. Not in the least intimidated by the reporters, camera crews and dignitaries at the back of the room -- who outnumbered them 2 to 1 -- they kept stealing Bennett's lines.

"Let me just get it out," he beseeched them once as they raised their hands and volunteered answers before he had finished his questions.

Bennett told the class that while segregation hurt black people most, it also made white people "less than they should be. Martin Luther King was working for the moral improvement of whites" as well as the equality of blacks.

After nearly an hour of banter and instruction, Bennett gave the class an A-plus. They gave him a medallion and a King T-shirt signed by class members. "And you remembered Extra-Large!" he said as he thanked them.

Afterward Sir Jason Green, 10, a thin, serious, bespectacled boy with the makings of a critic, gave Bennett a B. He explained, "There were some answers he didn't know" -- among them, how long it took to arrest James Earl Ray, who was convicted in King's death. Ray was arrested on June 8, 1968, about two months after the April 4 assassination.

At a news conference later, Bennett said he saw no conflict between his embrace of King's teachings and his support for the Reagan administration's efforts to phase out affirmative action, which have drawn criticism from some civil rights leaders.

While "people of good will disagree about the means," he said, "I don't think anybody disagrees about the ends," a colorblind society. "I think the best means to achieve the ends of a colorblind society is to proceed as if we are indeed a colorblind society . . . . I think the best way to treat people is as if their race did not make any difference."

As for the administration's proposal for a voucher system giving parents the right to chose among public, parochial or private schools, he said, "The group that supports our proposal most emphatically are inner-city, poor parents."

Later in the day, several hundred students from Georgia and Alabama gathered at the King Center's Freedom Hall auditorium for a "teach-in" by dignitaries, including Bennett; King's sister, Christine Farris; Jean Young, a teacher and chairman of a task force on public education created by her husband, Atlanta Mayor Andrew Young; the state and city schools superintendents, and Mary Hatwood Futrell of Fairfax, Va., president of the National Education Association.

In keeping with the spirit of unity, neither Futrell nor Bennett referred publicly to his recent criticism that teachers-union leaders make their members sound like "whiners" who do not like children and are concerned only for their paychecks.

Futrell said later that the two had not discussed the matter "yet." Bennett, seated on stage, was the first to rise from his seat and applaud after Futrell's remarks, in which she warned her young audience, "My fear is that we are in real danger of reducing (King's) teaching to meaningless platitudes . . . . It's up to you to see that we thwart the threat of apathy."

The day's scene-stealer was a brilliant patchwork quilt made by students in the 35 schools nationwide that bear King's name, including one in Beltsville, Md. One student described it as "a collage of our history" made from "scraps of fabric." Also present to take bows were the veteran quilters who had stitched together the "dream quilt.