Thousands of students are expected to walk out of their classrooms in protest of gun violence Friday, the 19th anniversary of the Columbine High School massacre that left 13 people dead in 1999. It is the second major national school walkout in response to gun violence since a shooting at a Parkland, Florida, high school earlier this year. Walkouts are planned at 2,000 schools around the nation, with at least one in every U.S.