High school students across the United States are planning to walk out of classes all day on Friday as part of the National School Walkout, a protest against what participants see as political inaction in the face of mass shootings and gun violence.

The event, which grew out of a petition on Change.org, comprises more than 2,000 walkouts nationwide, with at least one planned in every U.S. state and the District of Columbia.

With thousands of students expected to participate, here’s what you can expect from Friday’s walkout.

When is the walkout?

The National School Walkout is being staged on April 20, a date intended to mark the 19th anniversary of the mass shooting at Columbine High School in 1999. At the time, the Columbine shooting, in which 13 people were killed, was the deadliest school shooting on record.