Witnesses have recounted the terrifying scenes at Marjory Stoneman Douglas High School in Parkland, Florida, after a gunman killed at least 17 people.
Live television footage showed students streaming out of the building as the shooter, believed to be armed with an AR-15 rifle, fired at teachers and students.
Student Jeiella Dodoo told CBS News she and her schoolmates had evacuated their classroom calmly after hearing what they thought had been a routine fire alarm.
"The alarm went off so we had to evacuate from our classes," she said.
"Then we heard gunshots. I heard about six gunshots.
"And then some people started running and then everyone started running because we were like, 'If it's real, then just run.'"
Teacher Melissa Falkowski told US networks she had helped 19 students squeeze into a closet with her.
"We were in there for probably 40 minutes. We were locked in the closet until SWAT came and got us," she told CNN.
A former student recounted the events after speaking to witnesses.
"Kids were freaking out. Some kids froze. Some kids were on their phones. A lot of were on their phones just trying to snap chat everything. Because they thought it was a joke and it wasn't. There's kids freaking out. Students freaking out. Teachers. It sucked," she told CNN.
Students from the Marjory Stoneman Douglas High School in Parkland, Florida, evacuate the school following a shooting. Source: WPLG-TV
Kyle Yeoward, a 16-year-old junior, said he was in the bathroom on the second floor of the building on the campus where most of the shooting took place when he heard two shots.
"He let loose on the freshman building," Kyle said
Student Hector Navarro told CBS News more than 100 students were trying to flee the shooter after they realised it was not a drill.
"We are in disbelief honestly. We started to run. We were squished into this little area to try to get through. More than 100 kids, all trying to get through at once," he told CBS News.
"I hopped the fence to get away. Started running through the bushes and stuff."
McKenzie Hartley, 19, who identified herself as the sister of a student at the school described the scene in a text message: "She heard him shooting through the windows of classrooms and two students were shot."
The shooting took place at Marjory Stoneman Douglas High School in Parkland, a city in the southern part of the state between Boca Raton and Fort Lauderdale.
The suspect was identified as former student Nikolaus Cruz, 19, who was expelled from the high school, Sheriff Scott Israel said.
The shooter is believed to have used an AR-15 rifle and killed 12 people inside the school, two just outside, one in the street and two died in hospital.
