JAMARCUS MARSHALL, a 17-year-old high school sophomore in Mansfield, La., believes that no one should be able to tell him how low to wear his jeans. “It’s up to the person who’s wearing the pants,” he said.Mr. Marshall’s sagging pants, a style popularized in the early 1990s by hip-hop artists, are becoming a criminal offense in a growing number of communities, including his own.Starting in Louisiana, an intensifying push by lawmakers has determined pants worn low enough to expose underwear poses a threat to the public, and they have enacted indecency ordinances to stop it.Since June 11, sagging pants have...