Nonfiction -- briefly; The School Book, by Anne Bernays.New York: Harper & Row. $10.95.

Anne Bernays, whose last novel, "Growing Up Rich," won an award and considerable acclaim, writes with the kind of sharp humor and wit that make her sixth novel, "The School Book," ironic if not instructive. Bernays has a wonderful ear for the rhetoric of self-styled liberals and their stuffy opponents. Here she pokes fun at everything from Harvard to New York publishing. Would that every aspect of plot and characterization were as polished as Bernays' tone.

The events in "The School Book" are what its jargon-happy characters would probably call a "relevant, hands-on learning experience." Set in a swank, coed private school, the novel attempts to teach about life, love, and politics of higher education.

Sally Cooper, who narrates the drama-cum-lesson, is a trustee and confidante to Valerie Green, who has just become the Tigris School's first woman head. As their friendship develops, so does trouble -- a grading scandal, racial tension, unflattering magazine publicity, and a passionate affair (depicted in terms that leave little to the imagination) between Valerie and an instructor.

Sally, herself, seems to stretch credibility by being all-too- trendy. A stereotyped feminist, she believes that liberation consists of going without a bra and thinking of other women as "sisters," as if she'd just escaped form a Benedictine order. And despite her beliefs, she can't make a solid commitment to anyone -- Valerie, her boyfriend, or the students.

She does, however, involve herself in intrigue, and its momentum sustains the novel through the first semester. The rest of the book meanders as badly as the Tigris curriculum. Important plot questions remain unresolved, and readers never see Sally grow into the more sensible person she's now supposed to be. She finishes her education at Tigris a lucky escapee -- grasping not a diploma but a lot of loose ends.