WASHINGTON — Summoned by high school students swept up in school violence, thousands swarmed into the nation’s capital and cities across America on Saturday to march for gun control and ignite political activism among the young. Organizers of the “March for Our Lives” rally in Washington hoped their protest would match in numbers and spirit last year’s women’s march, one of the largest Washington protests since the Vietnam era and one that far exceeded predictions of 300,000 demonstrators. Bearing signs reading “We Are the Change,” ”No More Silence,” ”Keep NRA Money Out of Politics,” protesters lined Pennsylvania Avenue from the stage near the Capitol, stretching back toward the White House.