A little more than a week ago, Connor Dietrich was a normal high-school student whose daily routine included stressing out about standardized tests and going to afternoon practice for the diving team.

Now, he is a survivor of a mass shooting. His friend Nicholas Dworet and his brother's best friend Peter Wang are dead. And in their memory, Mr. Dietrich decided to travel to Florida's capital to push lawmakers to restrict access to guns.

"If Nick was alive, this is exactly what he'd be doing," Mr. Dietrich, 17, said on Wednesday. "I'm missing his funeral to be here for him."

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Mr. Dietrich was one of more than 100 students from Marjory Stoneman Douglas High School in Parkland, Fla., to make the 700-kilometre journey north to Tallahassee this week. Some were missing the funeral of a friend or had just attended one. But joined in grief and anger, they had resolved to use their voices to change the debate over guns in America.

On Wednesday, exactly a week after 19-year-old Nikolas Cruz allegedly killed 17 people at their school, the students walked the twisting corridors of the Florida state legislature holding meetings with dozens of state lawmakers. Outside, hundreds of students from local schools and universities joined a large rally in favour of gun control.

Eloquent and furious, the Parkland students have altered the script for responding to mass shootings. Days after the attack, they called on policy makers to take concrete action to prevent future tragedies and began working to turn their cause into a national movement. Their activism appears to be shifting the ground in Florida, where legislators are reportedly considering small but significant changes to state law (earlier in the week, however, Republicans rejected a move to consider a ban on assault weapons).

In the wake of the shooting, U.S. President Donald Trump has signalled that he might be willing to consider changes to federal gun laws – for instance tightening background checks and raising age limits for gun purchasers – but he has yet to put forward specific proposals. On Wednesday, he met with students and parents from Parkland at the White House for a "listening session." Mr. Trump said he would explore the possibility of arming school personnel if they had special training.

In Tallahassee, dozens of Parkland students emerged from the city's civic centre early on Wednesday morning. They had slept little, using the nighttime hours to research voting records and prepare their appeals to lawmakers and the public. They filed out carrying pillows, sleeping bags and blankets, all wearing some kind of school T-shirt – from the orchestra, the softball team, the masquerade troupe, the newspaper.

Everywhere they went, they encountered expressions of support. The night before, when they arrived in Tallahassee, their buses were greeted with applause from local high-school students who also carried signs urging them forward. The sight moved Lorena Sanabria, 16, to tears. "I was sobbing," she said. Coming to Tallahassee and agitating for change is "definitely how we cope."

The students seized their chances to press lawmakers. In a committee room on the fourth floor of the Capitol building, they pushed three Republican state senators to back changes that would have prevented the gunman who killed their friends and teachers from buying a semi-automatic rifle. Olivia Feller, 16, rose and asked a pointed question of Joe Negron, who leads the state Senate: Would he support raising the minimum age to purchase an assault rifle to 21, yes or no?

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Mr. Negron waffled. Before making a decision, he wanted to see the full text of a bill. But Bill Galvano, his Republican colleague, was unequivocal. "My answer is yes," he said. "Thank you!" the students yelled and burst into applause.

Later, Ms. Feller went outside to see the simultaneous gun-control rally being held on the Capitol grounds. The local school district had said it would excuse the absences of students who wished to attend the protest. "It's so surreal," Ms. Feller said, surveying the crowd of more than 3,000 people.

"I was just a regular student a week ago. Now we're right in it."

At an emotional midday news conference, nine Parkland students rejected the critiques and conspiracy theories that have been hurled in their direction – that they were too young to know what they were doing, that they wouldn't last long, even that they were "paid actors" according to one right-wing website.

Lorenzo Prado, an 11th grader, broke down crying as he remembered his murdered friends and the experience of being held at gunpoint after police officers mistook him for the shooter. As Mr. Prado recounted his story, three of the other students set to speak wiped tears from their eyes.

"The law has failed us," Mr. Prado said. The gunman was able to purchase an assault weapon "before he could drink beer."

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Sofie Whitney, 18, said she was discouraged by her interactions on Wednesday with lawmakers, but warned them not to underestimate the assembled students. "We will not let those 17 beautiful souls die for nothing because we are going to make a change," she said. "We will not give up. This is only the beginning of our history. Please be on the right side of it."