Audience involvement was raised to an art last night during the Back-to-School Boogie at the Capital Centre, where a sellout crowd of about 20,000 was more than happy to work almost as hard as the performers they paid $10 to see.

The night's lineup supplied a free-form mix of chants and rapid-fire, rhythmic raps built on a foundation of complex yet repetitious percussion work. Occasionally, a melody might be discerned here or there. The total effect might have set the uninitiated to wondering, "Is this really music?"

But it undoubtedly is music, fully involving the listeners, who seemed defenseless to its urgings to dance and chant along.

D.C.'s own "Experience Unlimited," more commonly known as EU, quickly established an amazing dialogue with its young audience. Almost without fail, the audience answered the group's every phrase with an appropriate counter-phrase.

EU: "Short, big, fat, tall."

Crowd: "We all can get on."

When E.U. sang out "Oh, la, la," the audience, of course, returned the phrase with the unamplified power of thousands of voices.

If only algebra American history could be learned with equal zeal.

The climax of the night's rapping delights came when six men who call themselves Grandmaster Flash and the Serious Five concluded their performance with a poignant piece of prose about a ghetto generation trying "not to lose its edge," called "The Message." One curiosity about Grandmaster is that its live act is hardly live at all: There there are no musicians and the group sings to recorded music.

All the shouting about "racking it on down" and "getting up to get down" wore thin by the time another local group, Trouble Funk, appeared halfway into the show. The fault was not with the group, which has a highly polished stage act and real vocal talent. Too bad none of these qualities is necessary to make rap music.

The Sugar Hill Gang, which first popularized rap music, performed in place of Cameo.