THERE, IN the middle of McPherson Square before the lunchtime crowd munching on salami sandwiches and potato chips, was the high school chemistry teacher singing lead vocals in a teen-age band. What did he think he was doing?

"Making their day," said Kenneth Howell, the teacher whose seven-member soul group, the B.C. Band, is one of 10 local acts participating in this year's "Munch for Lunch" program, arranged by the D.C. Department of Recreation.

As part of the department's "Showmobile," which has presented dozens of free concerts since 1968, the program offers an hour-and-a-half of noontime diversion three times a week at three locations in the city through August: Rawlings Park, 19th and E streets NW, on Tuesdays; McPherson Square, 14th and K streets NW, on Wednesdays; and Dupont Circle on Thursdays.

The 10 participating groups, ranging from Southern Wind, a rock group, to The Ambassadors, a "big band" orchestra, rotate from site to site in what has become a summer tradition in the city and a chance for the groups to get much-needed exposure.

"Some of the groups go to the big time after this," said Raymond Gray, music coordinator for the Department of Recreation, referring to the recordings made by such local soul groups as Experience Unlimited and Rare Essence, which acquired a loyal following as Showmobile participants.

Certainly, said Kenneth Howell, the B.C. (for Big City) Band is shooting for the same recognition. "Of course we want to become a recording group--like the Commodores," he said, noting a contract-in-the-works with Columbia Records.

But in the meantime, the band members--mostly former students at Spingarn High School, where Howell teaches chemistry--spend their time rehearsing their repertoire of Top-40 hits in the basement of Howell's Northeast Washington row house and trekking to gigs at small, smoke-filled clubs in Howell's Chevy van.

Howell's brief tenure with the B.C. Band started by chance when he was "discovered" last March at Spingarn's faculty-student talent show. Crooning an old Temptations' classic, "My Girl," Howell was spotted by vocalist Frederick Gartrell, who saw him as a prime candidate to fill the 7-year-old band's vacant manager slot.

"I told him I'd think about it," Howell said, but soon found himself at the group's next rehearsal, called a vote by band members on whether they wanted him, and "here I am." Howell then traded his post as head football coach at Spingarn for the three week nights of rehearsal and continual booking duties as manager of the band.

During the school year, Howell said, the band plays at high school dances and at "as many proms as we can," performing everything from "Ebony and Ivory" to "Moon River." "We have to play those 'oldies but goodies' for the older people," he said. "They're the ones who'll put out the money to see us at the clubs."

It's the occasional reunions and conventions that bring the highest yield, but, says Howell, the $200 the group takes home for each "Munch at Lunch" show isn't bad either.

"In fact, it's pretty good considering you're only working an hour and a half and 400 people are listening to you," he said. "Things can get pretty dull when you're walking in circles looking for a place to eat. Our job is to put 'em in a good mood before they go back to work."