Police surrounded the school in Parkland, Florida after as the gunman opened fire.
Between 20 to 50 people are thought to have been injured, and the gunman has now been caught by police.
Margate Fire Department told CBS Miami there are between 20 and 50 victims while a local fire chief told the Miami Herald at least one person has been killed.
Pupils barricaded themselves inside classrooms as SWAT teams descended on the school.
Speaking to ABC news, a mother gave a harrowing account as her daughter was still inside the school.
Florida School Shooting: A mother revealed that her daughter was scared in the school
She said: “Yes, she’s texting me right now, she is saying, she keeps telling me to stay away. ‘Be safe mom you stay away’
“I’m telling her no, Crystal, I’m your mother, I am not staying away.
“I was actually at the nail salon and I saw Stoneman Douglas High School, I thought, oh my God, that’s my daughter’s high school
“I ran home and turned the news on and my husband said maybe you should go see because they were letting some of the students out but she is still in there.
“She had text me fairly close to when it happened. She said she is scared, she heard the gunshots.
Yes, she’s texting me right now, she is saying, she keeps telling me to stay away
“She says, ‘I don’t want to go to school tomorrow mom’ and I said don’t worry about it.
“She’s just been staying in the classroom, and telling her to do what the teachers are telling you to do. And I said ‘can I call you’ and she said ‘no the teachers are telling us to stay quiet’.”
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.
Coral Springs Mayor Skip Campbell said moments later there have been “multiple fatalities.”
The suspected gunman is reported to have been arrested around a mile away from the school premises.
Police believe the suspect is a student at the school and said he was wearing a hoodie and has red hair and freckles.
He is also believed to be a member of the US Army Junior Reserve Officers’ Training Corps program.
The White House confirmed President Trump has been made aware of the incident and he has since 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.”