She has been at the crest of every jazz movement in this century, and as she nears 70 she could take the liberties of a doyenne, slowing and pontificating. But Mary Lou Williams keeps on keeping on.

In testimony to her own endurance, the American pianist, composer and arranger has packed her schedule this week with three of her favorite pursuits.

Last night at Ford's Theater she gave a concert of sacred music with the Howard University Chorale.

Earlier, at Howard, her rehearsals became laboratories for an art form, as Williams the professor emerged. And at the same time, she managed to keep in touch with her office at Duke University, where nearly 200 students registered for her classes.

Finally, she paused to lament over a favorite theme: the abandonment of jazz by blacks under 35.

"Blacks are in darkness about what is happening to this art. Jazz came out of a race, a race of suffering. When you are complacent, you just don't have the feeling. It's almost a return to the time jazz was a derogatory word," says Williams. A scowl passes over her face of mahogany satin, and she pulls the hotel's red bedspread around her as if chilled. "Everybody under 35 has lost it."

Mary Lou Williams has every right to be mad because she hasn't lost it. Her own life has paralleled the development of jazz. As a young child in Atlanta, she was introduced by her mother to ragtime and the blues. When she moved to Pittsburgh, where she spent most of her early years, she was given money by the males in her family to play classics such as Il Trovatore, and also introduced to the gin joints. Her first gig at a boogie-woogie club, where she was taken surrepticiously by her stepfather, happened before she was 10.

By the time she was a teen-ager Williams was swinging in Kansas City and Chicago with the godfathers of jazz, such as Art Tatum and Andy Kirk. Through the swing and be-bop periods she arranged for the masters -- Duke Ellington, Jimmy Lunceford, Benny Goodman and Dizzy Gillespie -- and coached Bud Powell and Thelonius Monk in her New York apartment.

In the late 1940s she arranged the first jazz piece for a symphony, the New York Philharmonic. Then, ironically, at the height of the modern jazz era epitomized by Miles Davis, Williams retreated from performing. In 1956 she converted to Roman Catholicism and reemerged with a new form: liturgical jazz. She found a new audience and new manager, a young Jesuit priest. One of her best-known contemporary works, "Mary Lou's Mass," was commissioned by the Vatican. At last night's concert, sponsored by the National Office for Black Catholics, Williams was given a humanitarian award named for the late congressman, Ralph Metcalfe.

In addition to her busy nightclub and concert schedule, Williams takes a special interest in trying to rescue young people from the Funkadelic syndrome. She takes the youngsters in her Harlem neighborhood for rides in her white Cadillac, buys them ice cream and puts on some jazz.

"I usually start them on a little Ben Webster," says Williams, referring to the tenor saxophonist best known for uncomplicated, majestic ballads. "Then I compose some rock things on the piano. You know, black people invented rock and dominated it until it became commercialized and slop. And the kids do react. One came back and told me he had bought a jazz record, 'Bitches Brew' by Miles," and she stops, laughing at his avant-garde selection. "I told him he went beyond me."

That downplaying is part of her posture -- she began a conversation before her Tuesday evening rehearsal at Howard by announcing, "There will never be another Art Tatum, Fats Waller or Bud Powell" -- but actually she is very conversant with the 1970s jazz fusion artists. When the Howard students gathered around her piano and asked for evaluations of McCoy Tyner, Herbie Hancock and Sunny Liston Smith, she was well versed.

While the rehearsal was in progress, Williams exhibited a stern and patient motherly side. Fanning herself with a funeral home fan in her right hand and playing with her left, she tried to achieve some spontaneity in a repetitive phrase. "Do it like this," she said, creating a zigzag pattern with her hands. On the third try, she demonstrated by chopping. As the 55 voices reached the rhythm, Williams closed her eyes and played a semi honky-tonk beat. She was all smiles.

Black students are a particular peeve of hers, though none of the tensions she describes with her students at Duke surfaced during the rehearsals at Howard. "When I first got to Duke the black students all sat by themselves in the cafeteria, all studied together. I would tell them you are too young to know, you haven't seen racism, but they wouldn't listen to me for the first year," says Williams, who has been at Duke for two years and has just had her contract renewed for three more.

In her sessions at Duke, the students often called her a Tom, criticizing the white audiences her contemporary music attracts. "And I would tell then, "Because they know what's happening, you dumb son of a b -- . Send your Al Capone (leader) to me.' And I had to prove to them that they were the Toms," says Williams. One of the anecdotes she used to rebut students' notions that the stars of '40s and '50s were Toms concerned Stepin' Fetchit, the actor whose name and screen image personified the head-scratching, grinning darky.

"One night Stepin' came into a club where I was playing with Andy Kirk. Now the gangsters were looking for him and Chicago had the worst gangsters in the world. Stepin' put his feet on the table, took out a bottle of champagne, and just sat back. When the gangsters came in, they surrounded him and Stepin' jumped up, pulled out two guns. And I hit the floor. Now that's not a Tom," recalls Williams, gleefully.

Besides her sense of mission to save jazz, her determination is fueled by her spiritual beliefs. During a rough period, she had turned to the church. "I have always felt everyone should go to church. But most religious didn't appeal to me because I didn't understand the ideas. I was always taught that God doesn't hold any secrets," says Williams. "So it was the straightforwardness and the access -- the churches were open in the daytime."

The combination of continuing creativity and inner peace has only enhanced her reputation. She carries the same weight and wholesome look she has had for years, her high cheekbones energizing her face. Only black circles around her eyes indicate her age. "I feel very strong and I'm thinking better. I think religion has eliminated the confusion around me, I always seem to be creating," says Williams.

"when the people at Duke asked me to stay for three more years, they said I couldn't be replaced. Well, I don't know but I'm going to be around trying to help these young people for a while."