... [S]o-called flash mobs have taken a more aggressive and raucous turn here as hundreds 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. On Wednesday, the police here said that they had had enough. They announced plans to step up enforcement of a curfew already on the books, and to tighten it if there is another incident. They added that they planned to hold parents legally responsible for their children’s actions. They...