Audition week at PS 156

''I dream of feet,'' says a teacher from the New School of Ballet. It's audition week, and the NSB is combing public schools in New York City's less affluent neighborhoods for children whose feet can point at a 180-degree angle, among other qualifications, but who can't afford ballet class. The owners of the right feet get a free semester of classes and free dance clothes, to see for themselves if they want to be dancers.

PS 156 in the Bronx, a tiled fortress roaring with children, is a likely repository of alignment, flexibility, coordination, elevation, musicality, and spirit, other qualities the NSB looks for. NSB teachers and staff, toting ballet bags stuffed with paperwork and practice clothes, and scented soap to wash up with after handling feet, have straggled up from the subway, looking like some kind of guerrilla expedition as they enter the corridors of the school.

But they fit right in. Acting principal Alice Wells welcomes them, the children have been told about ballet, and they're lining up to try out. ''We have found teachers are very willing to give children a new experience if approached with respect,'' says Georgia Delano, administrator of the school. She's amazed that the program works so well, considering how ''we do upset the schools' schedules.'' But if you add Mrs. Delano's tact to the speed and aplomb of the staff, and the students' willingness to hold out a foot and take a jump for a chance at ballet, you have an unbeatable combination. ''The audition is designed to give the child an interesting, good time, even if he's not chosen,'' she says.

Second and third graders file into the gym in small groups and stand on a line. Paul Sutherland, who, before teaching at NSB, danced with American Ballet Theater and was associate balletmaster for the Joffrey Ballet, takes each child's right foot gently in hand, as if they were skittish horses waiting to be shod, and bends it into a point. The staff leans forward behind their table as each foot bends. One is very supple. He darts a significant look at NSB associate administrator Katherine Moore. ''Is that a foot or is that a foot?'' she says with quiet enthusiasm, and marks ''definite'' on the card.

Just as quickly, she marks ''reject'' for others.

They used to take more children whose physical attributes were ''borderline, '' she says. ''Those children had the most difficulty. It was unfair to even show them the place (the Feld studio). . . . We've reached the point where we're very severe with ourselves.''

''We also look for the child who has musicality and has a love and has a light in their eyes . . . ,'' says Cora Cahan, assistant director of the Feld Ballet. Looking for that is the fun part of the audition. The children change into New Ballet School leotards and trunks and prance, skip, and leap. The eyes do light up. Auditioners smile over ambitious moves. Final choices are made. ''She's on the track team, don't steal her,'' pleads a gym teacher who has been looking on as a merry third grader bolts across the gym. ''She's the fastest girl in the school for her age.''

The results are tallied, and invitations to the NSB go out to 18 more children who will spend this term finding out if, perhaps, they are ballet dancers.