At a time when arts education is considered a luxury and building prisons is regarded as the only practical response to crime, a company like the Mosaic Youth Theatre of Detroit does double duty. It not only provides talented, eager, disadvantaged kids with increasingly rare writing, directing, and performing experience, it also provides a clear-eyed view of lower-class life that's in marked contrast to the Minicam melodramas of local news and the rhetoric of politicians. Who Killed Johnny Maze? has been written entirely by its cast of 12- to 20-year-olds, under the direction of Mosaic founder Rick Sperling; and this whodunit, which follows a teenage girl's search for the murderer, proves a moving, frank, complex portrait of dysfunctional America. One by one we meet the people in Johnny's life--an alcoholic mother, drugged-out friends, a manipulative, narcissistic girlfriend, and a heartless rival gang member--until finally it's clear that everyone Johnny knew contributed, each in his or her own way, to his death. Though from time to time these young performers lapse into over-the-top acting, Who Killed Johnny Maze? reveals surprising finesse and the kind of energy and conviction missing from all too many adult theater companies. Theatre Building, 1225 W. Belmont, 327-5252. July 7 and 8: Friday, 10 AM and 1 PM; Saturday, 1 PM. $15; $10 for college students and seniors, $7 for persons 18 and younger.