Students walked out of class at 10 a.m. Friday in each time zone to observe a moment of silence for shooting victims; many are then moving on to other actions, such as rallies in the larger cities or volunteer work.
Before Friday's walkouts began, the latest school shooting happened in Ocala, Florida, northwest of Orlando. Police said a student was shot in the ankle at Ocala's Forest High School
, and a suspect is in custody.
Friday's walkouts, while drawing momentum from February's mass shooting at South Florida's Marjory Stoneman Douglas High School
, also marks the 1999 Columbine High School massacre
in Colorado in which two gunmen killed 12 students and a teacher.
Many students across the country say not enough has been done to help prevent mass shootings. In Florida, Stoneman Douglas freshman Ryan Servaites pointed to Friday's shooting in Ocala.
"The fact that it happened on this day, in a way, reinforces what we are trying to get across," he said.
"Enough is enough. Children are dying. Children are being hurt. We won't stop. This is why."
Besides observing moments of silence, other actions during the day have included marching to a local lawmaker's office, allowing open-mic time for students to share concerns and helping register those who are eligible to vote. Students at more than 2,500 schools were to participate.
In Washington's Lafayette Square across from the White House, high school students gathered well before 10 a.m.
One of those demonstrators, Hiam Baidas of Falls Church, Virginia, said the country needs laws making it more difficult to buy guns.
"Right now I'm 18 years old, I live right across the street from Walmart, and I can go buy a gun -- and I don't think that's OK," she said.
"I think the youths are the movement that is going to change and better our country."
In New York's Washington Square Park, demonstrator Arielle Geismar, 16, said students will be persistent until gun laws are tightened.
"I've grown up in the generation of students who are realizing that we have lockdown drills all the time. We live with the constant fear that there is the possibility of a school shooting. No one should have to live like that," said Geismar, co-founder of the gun control activist group NYC Says Enough.
Hundreds of high school students gathered in Chicago's Grant Park to hear speeches.