In the aerial footage, a SWAT team can be seen moving towards the high school.
Dozens of police vehicles and a tank are also at the scene responding to the incident at the school in Parkland, Florida.
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“Here’s what we know so far: deputies are responding to reports of a shooting at Stoneman Douglas High. There are reports of victims. PIO will be on scene 3:15pm.
“Avoid the area of Stoneman Douglas HS. #BSO is currently working a developing incident regarding a report of active shooter.”
The gunman was believed to be wearing a gas mask and opened fire at around 2pm local time (7pm GMT). At least 20 people are thought to be injured.
President Donald Trump tweeted: “My prayers and condolences to the families of the victims of the terrible Florida shooting.
“No child, teacher or anyone else should ever feel unsafe in an American school.”
In a statement Senator Rubio said: “We are monitoring the horrible unfolding situation in Broward County, Florida with reports of a shooting at a local high school.”
Parkland is about 15 miles (24 km) west of Fort Lauderdale.
Local FOX-10 TV reported that five people were seen being treated by paramedics. Students were also seen walking away from the scene in single file.
Parents of students at nearby schools, including Sawgrass Banks Middle School and Westglades Middle School, said they received texts that those campuses were locked down.