THE 200 CHATTANOOGA HIGH SCHOOLERS want a song, and Tracie Spencer, here in Tennessee on a three-week tour of the South, is happy to oblige. "All right, all right," says Spencer cheerily. "Just let me spit out my gum."

Uh, yes, the gum. But then, Spencer is only 15, and her venue this afternoon is a classroom at the Howard School of Academics and Technology, where she delivers a stay-in-school/don't-do-drugs message along with the show.

Since she was voted Star Search's top junior vocalist in 1986, Spencer's Iowa-bred bubble-gum funk has produced two LPs and a handful of singles on both the pop and R&B charts. Her latest pop hit, a danceable summons to help the homeless, entitled This House, went to No. 3 this spring. "What trips me out is when I'm in a car with my friends and it comes on," she says. "They turn it up and start jammin'. It's fun."

That would be back in Waterloo, Iowa, where Spencer was born the youngest of four children to Marvin Spencer, a onetime backup singer for Jackie Wilson, and his wife, Theresa, a nurse. "When I was about 6 or 7, we started holding these little talent shows in our house called sing-along-soo-soos," says Tracie. "I would always be Diana Ross, my sister would be LaToya Jackson and my brother would play Lionel Richie."

A homemade demo tape done in a Waterloo library basement landed her on Star Search and eventually led to a Capitol Records contract. Success, however, has meant interruptions to her sophomore year at Columbus High School and to her social calendar. "In my whole life, I've had maybe three boyfriends," she says. "I mean, I'm always gone."

On the road, Spencer travels with a tutor, her dad and sister Tricia, 21, and calls her mother back home before bed each night. Although Mom "worries all the time," says Marvin, "we came to the conclusion that Tracie is talented, and we should let her do this if she wants." Does Dad have any regrets? Only one. "If it was up to her," says Marvin of his daughter, "we'd eat at McDonald's every night."