SLEEPWALK By John Saul (Bantam Books: $5.95; 464 pp . ). The lesson to be learned by East Los Angeles high school teacher Judith Sheffield isn't so much that, in the words of Thomas Wolfe, "You Can't Go Home Again," but in the fact that if you do you'll rue the day. In his new suspense work, "Sleepwalk," popular paperback novelist John Saul has the blackboard-jungle-weary Sheffield fleeing back to her New Mexico home town of Borrego to teach in more benign environs. Hah! Here we have a dusty little way-stop on the edge of the Kokati Indian reservation where the town's only viable employer, a small refinery, is on the verge of bellying up, and where something very odd is happening to its high school students--and various other people who, one way or another, seem to be questioning either activities at the school or at the refinery, where a corporate takeover is in the works. There is dirty work and hanky-panky in abundance in "Sleepwalk," and a finale that Cecil B. DeMille would have died for. The motivation does get a bit muddied, but there's more than enough cliff-hanging and blood-letting to make it immaterial.