By Holly Yan and Emanuella Grinberg, CNN

(CNN) -- In an unprecedented show of unity and political solidarity, waves of students marched out of class Wednesday to demand stricter gun laws and an end to school massacres.

The National School Walkout took place across the country at 10 a.m. in each time zone. The protest was sparked by last month's school massacre in Florida and fueled by years of anger about what many say are inadequate gun laws.

"This is not a matter of left versus right. This is a matter of public safety," said Cate Whitman, a junior at LaGuardia High School in New York. "We're all working together, which is something we haven't seen from the adults in a very long time."

Those participating have three main demands for Congress:

-- Ban assault weapons;

-- Require universal background checks before gun sales;

-- Pass a gun violence restraining order law that would allow courts to disarm people who display warning signs of violent behavior.

Students planned to stay outside for at least 17 minutes -- one minute for each of the 17 people killed at Florida's Marjory Stoneman Douglas High School exactly one month ago.

Students at Stoneman Douglas said they were overwhelmed by the nationwide support.

There's a "sea of people everywhere. You can barely see the ground," Stoneman Douglas student Sam Zeif said. "It really shows us we're not alone."

In Hoboken, New Jersey, students chanted, "I am a bullet-free zone," and some held signs that read, "Chalk not Glocks!"

Escorted by slow-moving police cars, students from Maryland's Montgomery Blair High School marched to a Metro station, where they boarded a train to the White House.

Students covered the area in front of the White House, chanting, "We want change!"

"History has its EYES on you," one student's sign read, though President Donald Trump wasn't scheduled to be at the White House at that time.

'WE WANT CHANGE': Scenes from the student walkout to demand stricter gun control laws

In New York, students warned lawmakers that they are the future -- and will soon be old enough to vote officials out of office if they don't pass tougher laws.

"We are the change!" they chanted.

Walkout goes global

From Israel to Tanzania, students across the globe also left their classrooms Wednesday in solidarity with the American students' movement. In some places, students talked with teachers about "how lucky they are" that guns aren't a part of their everyday lives.

Eduard Štrébl, a senior at Walworth Barbour American International School in Israel, organized the walkout on his campus.

"I'm from Prague, Czech (Republic), and I'm not American," he said. "But to see an epidemic of school shootings in a developed country when it's so easy to limit such things, to see that there is nothing being done against that, that inspired me to organize the walkout here."

Why some students disagree

Some students chose not to walk out with their classmates -- and for different reasons.

Austin Roth, a senior at Lapeer High School in Michigan, said he's "100% supportive of those who choose to be in the national walkout to show they care about the lives lost in Florida and every other school shooting."

"However, I am not supportive of those who use a tragic event to push their political agendas, such as gun control," he said.

Instead of walking out, Austin and other young Republicans from his school gathered in the cafeteria to voice their opinions.

Austin, 17, says he's a "staunch Republican" who carries a copy of the Constitution in his pocket.

"I do support federal background checks, (and) I'm not completely against raising the age to 21" to buy firearms, Austin said.

But he said he strongly disagrees with the notion of banning assault rifles, saying they can be useful when confronted with multiple burglars or other criminals.

"Guns are not the problem. The people are the problem," Austin said.

In Minnesota, 16-year-old Noah Borba said he didn't walk out because he doesn't fully support the movement.

"Because I have yet to have heard many good ideas, the movement seems too vague for my liking, and I would not like to associate myself with something I could end up disagreeing with in the future," said the Buffalo High School sophomore.

While it would be "pretty cool" if the country banned assault rifles, "I don't think logistically it's realistic" to get rid of all the assault rifles already out there, Noah said.

Not just about school massacres

For D'Angelo McDade, a senior at North Lawndale College Prep High School in Chicago, gun violence is personal -- but not because of a shooting at school.

He was shot in the thigh as he sat on his front porch last summer, leaving bullet fragments in his body, he said. So, D'Angelo took the lead in organizing his school's walkout Wednesday.

"Many of our community members and young adults have established a sense of hopelessness and normalized the suffering that comes with gun violence," he said. "But they're ready to see a change."

Penalties for walking out

Some school districts have said they will discipline students who participate in the walkouts.

In the Atlanta suburb of Cobb County, Georgia, the school district said it will take disciplinary action against students who walk out, citing safety concerns. Those punishment could range from Saturday school to five days' suspension, per district guidelines.

The prospect deterred some students, but not all of them, Pope High School senior Kara Litwin said.

"Change never happens without backlash," she said Tuesday. "This is a movement, this is not simply a moment, and this is only the first step in our long process."

Outside Walton High School in Cobb County, some parents stood Wednesday morning with signs reading, "Children Over Guns" and "We Demand Action!"