The conspicuous core of "The Violation of Sarah McDavid," a two-hour movie on Channel 9 at 9 tonight, is the horror of a teacher's rape in the classroom. But through actress Patty Duke Astin's sensitive portrayal of the teacher, the violence becomes more the vehicle for a strong story of personal growth than a crime saga.

In Benjamin Harrison High School, there is a mountain of problems. The principal, played with apt tightness by Ned Beatty, doesn't want any trouble -- especially the school's recent incidents of violence -- to mar his well-nurtured path to a downtown administrative post. Then there's the principal's natural antagonist, the teachers'-union official, a character who makes full use of the astringent candor of Vernee Watson. Into these tensions comes McDavid, whose lifetime dream has been to stand in front of a classroom of kids, knowing they couldn't care less about O. Henry, and convert them.

The film painstakingly shows McDavid's quick popularity and her personal indecisiveness. In school she wins the support of her students by playing cards and marble games, by having them write about their personal losses and having the girls who were caught smoking in the bathroom eat lunch with her on her classroom floor.At home she resists the marriage proposals of her live-in boyfriend, Eddie the engineer, patiently played by James Sloyan. When she gains her confidence, after the progress at school and a gift of contact lenses from Eddie that take away her dowdiness, she plans to get married.

Then the nightmare of rape reactivates her feelings of withdrawal. Even though violence is commonplace on television, this graphic scene smacks you in the gut with as much realism as her reactions. Still more concerned about litter campaigns than a drug hotline, the principal assures her he will handle the matter "internally," even though the rapist is not a student at the school. He announces that McDavid has been "roughed up." And McDavid goes along until she realizes that her rape and other violent incidents at the school have gone systematically unreported to police authorities.

The characters are obvious, the action is slow but the change is admirable. McDavid becomes an activist not right after the rape, but after a subsequent baseball bat attack on one of her students. And though consciousness-raising is certainly the action here, the superb subtlety of Astin moves the transformation above cliche. Unfortunately, the movie ends with the activist emerging as she goes public before the school board, while all the other conflicts are unresolved.