Kasky describes how he left his class towards the end of the day, as normal, to go pick up his brother Holden from the special needs classroom.
"As we exited the school, the fire alarm went off. And as we retreated to the parking lot, per fire drill procedure, we were told to run back inside," he recalls.
That's when everything changed.
After "hours of confusion and terror", the Kasky boys managed to make it home.
"Though we made it home, 17 people didn't. Those 17 people were murdered on the grounds of a school that has always felt like the safest place to be in a town that's been called the safest town in Florida," he wrote.
Kasky says he is not asking but, rather, "demanding" that action be taken.
"Why? Because at the end of the day, the students at my school felt one shared experience - our politicians abandoned us by failing to keep guns out of schools.
"But this time, my classmates and I are going to hold them to account. This time we are going to pressure them to take action. This time we are going to force them to spend more energy protecting human lives than unborn fetuses."
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Kasky adds the responsibility for the shooting does not lie with the shooter alone. "While the alleged shooter may have had several issues, he also lived in a society where Sen. Marco Rubio refuses to take responsibility for the role gun culture may have played in this tragedy," the student wrote, before blaming the NRA as well.
"And there is no denying that the NRA continues to donate millions of dollars to politicians at every level of government."
He ends his column by appealing to those of voting age to change things for those who can't.
"I'm just a high school student, and I do not pretend to have all of the answers. However, even in my position, I can see that there is desperate need for change - change that starts by folks showing up to the polls and voting all those individuals who are in the back pockets of gun lobbyists out of office."
Kasky's #NEVERAGAIN movement has been gaining momentum on social media, with numerous people coming out in support of his words following the tragedy.
Kasky had already gone viral with a Facebook post he wrote when he got home from school the day of the shooting.
