A gunman broke into a Missouri high school on Monday morning warning, “You are all going to die!” before fatally shooting a female teacher and a teenage girl, and wounding six others before police killed him in an exchange of gunfire.

The shooting was reported just after 9am on Monday at Central Visual and Performing Arts High School in St Louis, prompting hundreds of students and staff to run from the building. The school was immediately surrounded by dozens of police vehicles.

Police Chief Michael Sack said fast actions by a security guard, along with police officers who “ran to the gunfire” helped end the shooting before more people were killed or hurt.

He said the gunman was about 20 years old, but did not identify him and said a possible motive was not yet known.

Authorities did not name the victims, but the St Louis Post-Dispatch identified the dead teacher as Jean Kuczka. Her daughter said her mother was killed when the gunman burst into her classroom and she moved between him and her students.

“My mom loved kids,” Abbey Kuczka told the newspaper. “She loved her students. I know her students looked at her like she was their mom.”

TV reports said officers entered the area with guns drawn shortly after 9am. Crime tape was placed around the school and parents were directed to another school building to reunite with their children.

St Louis Schools Superintendent Kelvin Adams said seven security guards were in the school at the time of the attack, each stationed at an entrance of the locked building. One of the guards noticed the gunman trying to get in at a locked door, but unable to. The guard notified school officials, who contacted police.

“It was that timely response by that security officer, the fact that the door did cause pause for the suspect, that bought us some time,” Michael Sack said.

He declined to say how the man eventually got inside, armed with what he described as a long gun.

Central Visual and Performing Arts shares a building with another school, Collegiate School of Medicine and Bioscience.

Police told TV stations at the scene that six people injured included one person who suffered cardiac arrest, and the others suffered gunshot or shrapnel wounds.

No further details about the injuries were immediately released.

One student, 16-year-old Taniya Gholston, told reporters she was in a room when the gunman entered.

“All I heard was two shots and he came in there with a gun,” she said. “And I was trying to run and I couldn’t run. Me and him made eye contact but I made it out because his gun got jammed. But we saw blood on the floor.”

Another student, Nylah Jones, said she was in maths class when the gunman fired into the room from the hallway. The gunman was unable to get into the room and banged on the door as students piled into a corner, she said.

Teacher Raymond Parks was about to teach a dance class for juniors when a man dressed in black approached. At first, Parks thought the man was carrying a broom or a stick. Then he realised it was a gun.

“The kids started screaming and running and scrambling. He walked directly into the two doors and pointed the gun over at me because I was in the front,” Mr Parks said.

For some reason, Mr Parks said, the gunman pointed the gun away from him and let Mr Parks and the dozen or so students leave the room.

“That’s what I don’t understand. He let me go,” Mr Parks said.