[H]undreds of teenagers have been converging downtown for a ritual that is part bullying, part running of the bulls: sprinting down the block, the teenagers sometimes pause to brawl with one another, assault pedestrians or vandalize property. . . . The flash mobs have raised questions about race and class. Most of the teenagers who have taken part in them are black and from poor neighborhoods. Most of the areas hit have been predominantly white business districts. In the flash mob on Saturday, groups of teenagers were chanting “black boys” and “burn the city,” bystanders said.