WHEN WILLIAM HARRIS was starting out as an art teacher at Taft School, he tried sketching portraits of his pupils. ''You got to prove yourself with junior high kids. Get their attention.'' Profiles. Suddenly he began to see things in them.

They are a basic part of his work now. Like figures hidden in a puzzle, these open-mouthed human coastlines lurk in his airbrushed and penciled and stippled paintings. He calls them his muses. Some have rabbit ears. He doesn't ask himself why.

Most artists support themselves by teaching. For Billy Harris, 38, the two careers have melted together so profoundly that he can't tell where one leaves off. Nothing is more crucial to him than showing his students at Ellington that communication is power, that the world beyond high school won't be interested in their excuses, that they must stop waiting for life to begin.

"It's not enough to be able to draw and paint," he tells them. "There's a lot of people on the boardwalks doing the same thing, and they're not going anywhere. It's not a matter of having a gimmick. You have to master many techniques. You have to work."

One gifted student loved to draw cartoons but refused to learn anything new; he just kept talking about what he was going to do. "He was scared to move on and test himself. I said, 'Don't tell me, show me.' I confronted him: fish or cut bait. He quit school. Well, but he'd made a decision."

Harris loves teaching. He knows his students. He understands when he should just try to stay out of the way. He worries about the C and D students for whom he can do little. He can tell which part of the room the disruption will come from in each class.

"I'm tough with 'em. But if I take a kid apart, it's because he's taking the class apart."

He has been known to apologize to a student. He tells them communication is a key to racism, and if you can't communicate you're a nonperson. He wants them to learn self-discipline.

Sometimes it all gets to be too much. "It can really drive you crazy: There's no immediate result. It's like planting seeds. I think: I'm too talented for this." Then he answers himself ironically: "For what?" The time wasted, the administrative details, the demands that began to encroach on his home life . . . It got so bad at Taft that in 1974 he took off on a sabbatical, returned to Howard for a master's, finally committed himself to completing a body of work. It was a turning point.

Harris has exhibited at the Corcoran, Franz Bader gallery and other places, has a show next March at Gallery 10 and will lecture at a group show in November at Bucknell College. His paintings are full of metaphor. Figures jump over pyramids or are devoured by beasts when they fail. He draws breasts, which he reveres as a symbol of sexual maturity. "People who are doing well in their work have somehow resolved the sex problem. They have found out who they are." He may spray paint on a cloth, slap it onto the canvas, find shapes and meanings in it, orchestrate the smudges until the work says what he wants.

They are beginning to sell now, for around $500. The money always goes for supplies.

"When I'm stuck, I go to three dimensions. I turn out pieces on my lathe. You can't fool around with wood. It takes discipline." Often these abstract figures feature breasts, too.

Once when the money was low, he made some striking bowls from particle wood, plywood, Masonite and other scraps he found at Metro sites, sold them at a vendor's stand outside Howard for $30 to $50, "and I had more money in my pocket than I have right now." Eventually the university ran him off. Then he had a shop downtown near Central Liquors. It was also his studio.

"An artist needs a studio. It's a state of mind. Work all night, you're too tired to clean up the sawdust in the living room. So you have a studio. My wife understands this. She knows I'm going to tune her out sometimes. She's my friend, too." She teaches P.E. in Prince George's County. There is a daughter, nearly 16, by a former marriage, and a son, 4.

"Concentrating. Focusing. It's so important." He was serious about basketball (though he is under 6 feet) but realized he would have to cast that aside. There were other sacrifices. He used to sport a huge, fearsome Afro and beard, but it was a hazard around the lathe. It had to go.

"They used to call me Wolfman," he said, just a trifle wistfully.