Emergency services rushed to Marjory Stoneman Douglas High School in Parkland, Florida, after several panicked calls from students inside the school.
One man, matching a description given by children, has since been taken into custody and is believed to be a former student from the school.
According to the Daily Mail, that man is Nikolas de Jesus Cruz, 19. He's believed to have entered the school armed with what appears to be an assault rifle, and began shooting at just after 2pm.
The school has been placed into lockdown, with one student even sharing photos from inside a classroom.
Coral Springs Police have been updating people via Twitter as the story develops, with police warning anyone in contact with students inside the school to tell them to remain calm.
'Police are working an active shooter scene, and ask that if you are in touch with your student you ask that they remain calm and barricaded until police come to their room,' they tweeted.
It has since been confirmed that there is 'at least 14 victims', with students continuing to be transported to hospital. They did not specify whether this was fatalities or injured people.
However Florida Senator Bill Nelson told Fox News there were 'many deaths'
Footage has now emerged of students exiting the school in single-file lines with their hands above their heads to show they aren't carrying weapons.
'What happened was that the fire bell rang and we all thought it was a drill because we had already had a fire drill,' an unidentified student told WSVN 7 News.
'We thought it's another drill so we didn't take it seriously and then we heard pops from the other side of the school.'
Matthew Walker, a 17-year-old student from the school, told WFOR-TV that all his classmates 'knew it was going to be him.
'A lot of kids threw jokes around saying that he was going to be the one to shoot up the school,' he said.
'He was going class to class just shooting at random kids. Everything he posts (on social media) is about weapons. It's sick.'