They bowed their heads in honor of the dead. They carried signs with messages like "Never again" and "Am I next?" They railed against the National Rifle Association and the politicians who support it.

And over and over, they repeated the message: Enough is enough.

In a wave of protests one historian called the largest of its kind in American history, tens of thousands of students walked out of their classrooms Wednesday to demand action on gun violence and school safety.

The demonstrations extended from Maine to Hawaii as students joined the youth-led surge of activism set off by the Feb. 14 massacre at Marjory Stoneman Douglas High School in Parkland, Fla.

"We're sick of it," said Maxwell Nardi, a senior at Douglas S. Freeman High School in Henrico, Va., just outside Richmond. "We're going to keep fighting, and we're not going to stop until Congress finally makes resolute changes."

Students around the nation left class at 10 a.m. local time for at least 17 minutes — one minute for each of the dead in the Florida shooting. Some led marches or rallied on football fields, while others gathered in school gyms or took a knee in the hallway.

At some schools, hundreds of students poured out. At others, just one or two walked out in defiance of administrators.

They lamented that too many young people have died and that they're tired of going to school afraid they will be killed.

"Enough is enough. People are done with being shot," said Iris Fosse-Ober, 18, a senior at Washburn High School in Minneapolis.

While administrators and teachers at some schools applauded students for taking a stand — and some joined them — others threatened punishment for missing class.

As the demonstrations unfolded, the NRA responded by posting a photo on Twitter of a black rifle emblazoned with an American flag. The caption: "I'll control my own guns, thank you."

The protests took place at schools from the elementary level through college, including some that have witnessed their own mass shootings: About 300 students gathered on a soccer field at Colorado's Columbine High, while students who survived the Sandy Hook Elementary School attack in 2012 marched out of Newtown High School in Connecticut.