TAMPA — Susana Matta Valdivies has always been afraid of public speaking.

Now 17, she remembers how her stutter made her even more nervous to talk in front of classmates on her first day of school, shortly after her family moved from Colombia to Tampa. Valdivies still re-members the exercises drilled into her mind from years of speech therapy. Even now, as a junior at Marjory Stoneman Douglas High School in Parkland, Valdivies gets nervous whenever she gives a presentation in class, and finds herself running through the same speaking exercises she learned when she was 5.

But when she took the stage in Tampa's Kiley Gardens on Saturday morning to address a crowd officials estimate topped 13,000, Valdivies knew what she had to say was more important than fear or embarrassment.

She didn't stutter.

"I still remember the fear and shock that went through us as we heard gunshots in school, but the hardest part for me was walking out of school, being escorted by police with our hands up in a single file line," she said.

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"To everyone who said it was too early for us to talk about politics, it was never too early. Since Columbine High, 7,000 children's lives have been lost due to gun violence. We are not too early. We are 7,000 lives too late."

Saturday's rally was just the latest in a string of press interviews, protests and marches Valdivies' and her classmates have spearheaded in the wake of the February 14 school shooting that left 17 dead.

But to join students from 50 Tampa Bay area schools for Saturday's sister rally in the national March for Our Lives movement, Valdivies ditched her spring break plans, bought herself a $47 plane ticket, and stayed up until 2 a.m. perfecting her speech.

Their efforts yielded 11 student-led rallies across central Florida Saturday morning, erupting simultaneously throughout Hillsborough, Pinellas, Hardee, Polk, Pasco, and Manatee counties to join in solidarity with more than 800 marches nationally.

They all shared the same, simple message, Valdivies said: Ban automatic weapons and "bump stocks," devices that can speed up the rate at which semiautomatic rifles can be fired. Require background checks be performed before a weapon is sold. Strengthen mental health services in schools.

"Plenty of legislators are using mental health as an excuse for gun violence but I have yet to hear any proposed solutions regarding it," said Sam Sharf, 17, a junior at Plant High School. "The monsters who commit these senseless acts of violence still beneath all the madness are people too… Instead of training teachers with guns, we must train them to identify students who they believe may need help mentally so the school psychologists and guidance counselors can give the students the help they need."

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Tampa Mayor Bob Buckhorn stood backstage at Saturday's event, hugging and congratulating each student stepping off stage like a proud father. He was feeling like one too, he said.

The older of his two daughters, a 16-year-old junior in high school, wanted to be at Saturday's rally too, he said.

She wasn't with him Saturday morning, backstage with the politicians, security teams and adults.

She was somewhere in the crowd, ready to march with her friends.

His seventh grader wanted to be there too, but was out of town. The day after the Parkland shooting, Buckhorn was among the wave of frantic parents who rushed to pick up their children from Tampa's Academy of Holy Names, when an anonymous threat forced the school to shut down.

"No one is immune to that fear, not even at a private Catholic school," Buckhorn said. "I'm going to speak out as a father, as the mayor and as a gun owner. I believe in the Second Amendment. I own guns. But there is no reason why a hunter like myself would need an AR-15 … I think a majority of gun owners would agree there's nothing wrong with a waiting period; there's nothing wrong with back-ground checks; there's nothing wrong with prohibiting citizens from owning weapons of war."

Times staff writer Jonathan Capriel contributed to this report. Contact Anastasia Dawson at adawson@tampabay.com or (813) 226-3377. Follow @adawsonwrites.