High school students across the United States are planning to walk out of classes all day 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 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.

Participants in the walkout will be leaving their classes at 10 a.m. local time.

WHAT DOES IT ENTAIL?

Students are expected to exit their school buildings for the day. Organizers have called on people to wear orange, a color that has become associated with the gun control movement.

After they leave their buildings, participants will gather to observe 13 seconds of silence that are meant to honor the 13 victims in the Columbine shooting.

After that, further activities are up to the leaders of each school’s individual protest. National organizers have called for daylong activism. In a planning guide on the National School Walkout’s website, organizers encourage participants to hold marches, participate in voter-registration drives and set up speeches.

WHO IS ORGANIZING IT?

The idea for the National School Walkout came from Lane Murdock, a high school sophomore from Ridgefield, Connecticut, about 20 miles from Newtown, the site of the mass shooting at Sandy Hook Elementary School in 2012.

In the Change.org petition that Murdock started, she cited the mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida, in February as a catalyst for the event.

“It is time for teenagers to speak their minds and put their frustration into action,” she wrote.

Murdock has been working with a team of other students at her high school to coordinate the national protest. Individual walkouts at schools are being organized and staged by local students.

WHAT ARE THEIR GOALS?

The walkout’s website says the event has three goals: to “hold elected officials accountable,” to advocate “solutions to gun violence” and to encourage students to be more engaged in politics.

More specifically, the event seeks to push Congress to pass legislation to halt gun violence. Among the specific requests are bills to strengthen the background check process for gun purchases; a ban on bump stock attachments, which allow semi-automatic guns to fire like automatic weapons; and raising the minimum age to purchase assault-style weapons to 21.

Organizers are also encouraging participants to pressure state and local governments to enact similar laws. They are also seeking to increase voter registration among younger Americans in the hopes of electing candidates who support gun control.

DIDN’T WE JUST HAVE A WALKOUT?

On March 14, one month after 17 people were killed in the Parkland shooting, students across the United States stepped out of their classrooms at 10 a.m. to call for stronger gun control laws.

Students left schools by the hundreds and thousands, sometimes in defiance of school administrators and school boards. In some cities, including New York, participants in the walkout then staged protests or marches.

The walkouts in March were the first major nationally coordinated action of a student-driven movement for gun control that was sparked by the Parkland shooting. Still, many students did not participate, especially in more conservative and rural parts of the country where gun control is not in wide favor.

Ten days later, on March 24, demonstrators gathered in Washington for the March for Our Lives rally, as well as at more than 800 protests held around the world to call for action against gun violence.

WHY IS THIS ONE DIFFERENT

The walkout held last month was much briefer, scheduled to last 17 minutes, one minute to honor each of the Parkland victims. In practice, some students continued to protest for the rest of the day. This walkout Friday is intended to last an entire school day.

The National School Walkout also comes as attention to the Parkland shooting has faded somewhat. Despite efforts by the student-driven movement, neither Congress nor the White House has taken significant action on gun control and gun safety measures. Congress did pass a spending bill in March that included measures to provide grants for school safety and to improve reporting to the background check system for gun purchases.

Gun regulation advocates have seen victory in Florida, three weeks after the Parkland shooting, Gov. Rick Scott signed a bill with some of the limits students have been urging. But some have argued the legislation did not go far enough.

On their website, organizers for the National School Walkout wrote that they see their event as part of a larger push to advocate gun control and that the issues they are addressing need “constant attention if we hope to change anything.”

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