Two students and two teachers have been killed and at least nine others injured in a horror school shooting in Barrow Country, in the US state of Georgia.

Students heard shots firing at Apalachee High School on Thursday (AEST) during what US President Joe Biden said “should have been a joyous back-to-school season”.

Frightening and chaotic scenes unfolded as students ran for their lives to seek shelter at a stadium, as officers swarmed the grounds.

Barrow County Sheriff Jud Smith said the suspect, named Colt Gray, a 14-year-old student at the school, surrendered immediately when he was fronted by law enforcement agents.

The teenager, who was a student at the school, would be charged with murder and tried as an adult, Georgia Federal Bureau of Investigation director Chris Hosey said.

The suspect was speaking with investigators, but they declined to say if they knew what motivated him. They also did not say what type of gun was used in the shooting.

Earlier, Smith said: “What you see behind us is an evil thing today.”

It was the first shooting of the new school year in the US, a stark reminder of the threat of gun violence in schools and colleges across the nation.

Helicopter video from WSB-TV showed dozens of law enforcement and emergency vehicles surrounding the school.

Erin Clark’s son Ethan, a senior at the school, was writing an essay in class when he heard eight or nine gunshots. Ethan and his classmates used chairs and tables to try to barricade the door and hide.

“I’m so proud of him for doing that,” Clark said. “He was so brave.”

Erin said Ethan texted her about an active shooter, and she rushed from her job at the Amazon warehouse to the school.

“School shooting,” “I’m scared,” “pls,” “i’m not joking,” said the texts, according to CNN.

“I’m leaving work,” Clark replied.

“I love you,” wrote Ethan.

“Love you too baby,” replied his mum.

Students wait with others to be picked up by their parents after the shooting. Photo: Getty

With the main road blocked to the school, Clark parked and ran with other parents. Parents were then directed to a football field.

Amid the chaos, Clark found Ethan sitting on the bleachers.

ABC News quoted a witness, student Sergio Caldera, as saying he was in chemistry class when he heard gunshots. Caldera, 17, told ABC his teacher opened the door and another teacher ran in to tell her to shut the door “because there’s an active shooter.”

As students and teachers huddled in the classroom, someone pounded on the door and shouted several times for it to be opened.

When the knocking stopped, Caldera heard more gunshots and screams. He said his class later evacuated to the school’s football field.

Law enforcement and first responders at Apalachee High School. Photo: Getty

Traffic going to the school was backed up for more than a kilometre as parents tried to get to their children there.

Biden said that America “cannot continue to accept this as normal”.

“What should have been a joyous back-to-school season in Winder, Georgia, has now turned into another horrific reminder of how gun violence continues to tear our communities apart,” he said.

“Students across the country are learning how to duck and cover instead of how to read and write.”

Apalachee High School has about 1900 students, according to records from Georgia education officials.

It became Barrow County’s second largest public high school when it opened in 2000, according to the Barrow County School System.

-with AAP