Students from the high school where 17 people were shot and killed last month have created a grassroots movement over gun violence that has spread across the United States.

“#NeverAgain: Pick up a pen,” reads a tweet by @NeverAgainMSD, an organization of students using social media to raise awareness about gun violence in the United States. “Write a letter(s) (handwritten or typed) directed to the White House and the President.”

MSD stands for Marjory Stoneman Douglas High School in Parkland, Florida, which was attacked by a young man using an assault rifle Feb. 14. Since then, MSD high school students have rapidly turned that tragedy into an organized political movement.

“Include experiences and what you wish to see changed *Submit copies of the same letter or submit multiple letters 2) Put the sealed letter(s) in an envelope with your name and the words “White House” on the outside. 3) Leave the letters at your school’s designated drop box.”

MSD student activists have focused on the National Rifle Association, a highly influential political lobbying organization that defends the rights of Americans to carry guns.

Measuring impact

The students have had success in pressuring some companies to publicly cut their ties with NRA partnerships. Major American retailers like Dick’s Sporting Goods, Walmart, LL Bean, REI and Kroger, which sold assault rifles, said they would take them off the shelves.

Dick’s and Walmart said they would not sell weapons to anyone younger than 21, as well. Walmart also said it would remove items from its website that look like assault weapons, such as pellet guns and toys.

“To all companies who severed ties with the NRA, those personally affected by their influence on legislation thank you,” tweeted Emma Gonzalez, a student activist from MSD who spoke out soon after the mass shooting.

Despite the students’ success in drawing attention from retailers and the news media, their impact on lawmakers remains unclear.

On the federal level, the Republican-held Congress is considering a bill called STOP School Violence Act that grants $50 million to schools for training and reporting, but does not address gun control directly. In the Senate, a vote on gun control this week has been delayed. And although President Trump made comments last week that seemed to indicate he supported tighter restrictions on gun sales that have been opposed by the NRA, it’s not clear that it is leading to any substantive change in federal gun laws.

There have been stronger indications that some U.S. states may move forward with gun restrictions.Florida’s governor signed new legislation that tightens gun control regulations, and allows for armed school staff members.

Social media amplifies message

The students’ massive following on social media have emerged as a powerful tool in their movement. Gonzalez, or @Emma4Change, has more than 1 million followers on Twitter, more than @NRA’s 608,000.

David Hogg, another MSD student activist — @davidhogg111 — has nearly 430,000 followers, mostly since the Feb. 14 shooting. Their school’s #NeverAgain movement Twitter account, a hub of students organizing rallies, has about 140,000 followers.

That exposure has allowed students to raise money and received guidance from well-established organizations, like Everytown for Gun Safety and The Brady Campaign to Prevent Gun Violence, that started out as grassroots efforts against gun violence.

Celebrities like George and Amal Clooney, Oprah Winfrey, Stephen Spielberg and Kate Capshaw, and Jeffrey and Marilyn Katzenberg each donated $500,000 to the #NeverAgain activists and the March for Our Lives, according to a report from Time’s Money section.

“#studentsstandup and we will continue to follow. You are a beacon of strength,” wrote Josh Gad, an actor in “Book of Mormon” and “Frozen,” on Twitter.

“These kids are asking the hard questions and being persistent in a way that should have been done by the adults years ago. #studentsstandup,” tweeted Liz Froment — @LFroment — a business writer in Boston.

For all of the support they have received on social media, there are vocal critics.

Collins Idehen, a gun-rights advocate and attorney who hosts a program on NRA TV, criticized the students on a channel that claims an online audience of more than 1.3 million.

“We somehow have devolved so far in this country that we literally blame gun owners for these shootings...Y’all insist on only having the conversation with yourselves & waging war with #NRA. You’re not waging war against a machine. You’re waging war against people.”

Despite the criticism, a major measure of the student’s reach comes Wednesday, when students across the United States are planning to walk out or school and hold rallies in support of more restrictions on guns, to try to prevent the next mass shooting.