SHOCKED students have described the terror they felt when their classmate opened fire on them in a Texas high school.

Nine students and one teacher are dead and another ten are injured, including police who responded to a gunman opening fire on an art class while reportedly shouting "Surprise!"

One young girl tearfully recalled the moment she was told by a teacher, "Run! Run! Go as fast as you can", before hearing a series of shots in the school grounds.

"I shouldn't be going through this. It's my school. This is my daily life," the student told reporters.

Harris County sheriff Ed Gonzalez said most of those killed in the massacre at Santa Fe High School on Friday morning were students, with a school resources officer also reportedly gunned down.

He said the 17-year-old male suspect, identified on Friday afternoon local time as Dimitrios "Dimitri" Pagourtzis, had been taken into custody, and a second person was also detained. At least one police officer was injured, he added.

Student Tyler Turner told KTRK the gunman pulled the fire alarm and students began pouring out of classrooms.

A student named Paige said she hid backstage in an auditorium as the first shots rang out and called her mother, who told her to stay calm and follow teachers' directions. "I was very, very scared," she said. "It's been happening everywhere. I always felt eventually it would happen here, too."

Students knew something would happen

Texas Governor Greg Abbott said explosive devices including a molotov cocktail were found in the suspect's home and a vehicle as well as around the school and nearby.

The suspect used a shotgun and .38-revolver owned legally by his father, added Mr Abbott, calling the shooting "one of the most heinous attacks that we've ever seen in the history of Texas schools."

He said there were no "warning signs" about the suspect ahead of time, although the teenager did post a picture on Facebook of a T-shirt with the words "Born to Kill" on it.

The gunman reportedly intended to commit suicide but gave himself up to authorities, saying he didn't have the courage.

Students described chaos as they heard gunshots and a fire alarm at around 7.40am and began running for their lives. Several told local media the shooter appeared to have a shotgun.

Some were seen covered in blood as they ran from the school, while others were taken away on stretchers and aerial footage of the scene showed a body covered with a sheet on the grass outside.

The Santa Fe school district, 48 kilometres southeast of Houston, issued an alert this morning saying the area had been placed on lockdown as students were seen sobbing into the arms of their families.

The US President later said the attack was "absolutely horrific", during a prison reform event at the White House.

"My administration is determined to do everything in our power to protect our students, secure our schools, and to keep weapons out of the hands of those who pose a threat to themselves, and to others," he said.

Melania Trump tweeted: "My heart goes out to Santa Fe and all of Texas today."

One student told local television station KTRK that a gunman came into her first-period art class and started shooting. The student said she saw a girl with blood on her leg as the class fled through a back door.

"We thought it was a fire drill at first but really, the teacher said, 'Start running,"' she told the Houston TV station.

Another student said she heard a "boom" and ran to the forest, while junior Dustin Severin said he saw the suspect wearing a trench coat and army boots heading to the art room before he heard shots. He said teachers shouted at them to run and he climbed a fence and ran through someone's yard.

A 16-year-old boy who said he considered Dimitrios a friend said the suspect was interested in guns and war simulation video games, but had never about talked about killing people. Tristen Patterson said the teenager did not show signs of being bullied, but that he rarely talked about himself. He says Dimitrios would sometimes enter the classroom "acting a little bit down or sad. A little bit sluggish. ... But he never talked about why."

The University of Texas Medical Branch in Galveston was treating three patients with gunshot wounds. One was a middle-aged man who was in the operating room in a critical condition, another was an adult female and the last a person under 18, who suffered gunshot wounds to the leg.

Clear Lake Regional Medical Center said it admitted seven patients, and Mainland Medical Center received two.

Aerial footage outside the school showed police escorting lines of students out of the building and searching them for weapons as police cars and ambulances gathered at the scene.

Local and federal law enforcement from around the Houston area, including agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives, responded to the incident.

Santa Fe school district superintendent Leigh Wall said in a statement: "It is with great sadness that I stand here to share with you that we experienced an unthinkable tragedy at our high school this morning.

"We are grieving the loss of members of our SFISD family."

Survivors of February's shooting in Parkland, Florida, tweeted their condolences to the students at Santa Fe High School.

It is America's deadliest school shooting since the Florida massacre, when 17 were killed at Marjory Stoneman Douglas High, whose students led the historic March For Our Lives gun control protest in March.

It is the third school shooting in the United States in the past eight days, with 22 reportedly since the beginning of the year.

Santa Fe is a city with a population of around 13,000 people.

- With wires