U.S. students walk out again to protest gun violence

By Keith CoffmanLITTLETON, Colo. (Reuters) - Demanding an end to gun violence and tougher restrictions on firearm sales, thousands of students again walked out of classes across the United States on Friday in hopes of putting pressure on politicians ahead of November's midterm elections.Timed to coincide with the 19th anniversary of the Columbine High School massacre, students left classes atmidmorning, many waving placards with slogans including "I should be worried about grades, not guns," and