Another described seeing the shooter wearing a trench coat and armed with a sawed off shotgun before barricading themselves inside their classroom.
Ten people were killed and 10 others wounded in the shooting at Santa Fe High School south of Houston, Texas Gov. Greg Abbott said.
Harris County Sherriff Ed Gonzalez says one person is in custody and a second person has been detained. The suspect has been identified as 17-year-oldlaw enforcement sources told CBS News.
Gonzalez says the majority of the dead are students. Gonzalez says a police officer is among the injured but the extent of the officer's injuries is unknown.
Michael Farina, 17, said he was on the other side of campus when the shooting began and thought it was a fire drill. He was holding a door open for special education students in wheelchairs when a principal came bounding down the hall and telling everyone to run. Another teacher yelled out, "It is real."
Students were led to take cover behind a car shop across the street from the school. Some still did not feel safe and began jumping the fence behind the shop to run even farther away, Farina said.
"I debated doing that myself," he said.
Student Damon Rabon told CBSN he was one classroom away from where the shooting happened in the art hall during first period. He said he was in shop class when he and other students heard "this loud banging, kind of ringing noise." He said at first they thought a desk fell or someone hit the shop door.
Then, they heard the same noise three or four more times.
Rabon said the substitute teacher went out and looked and saw the shooter, who he described as a short male wearing a black trench coat carrying a backpack and armed with a sawed-off shotgun. The teacher got everyone back in class and told everyone to get down and they barricaded the door.
The student said the substitute teacher pulled the fire alarm in the hopes that other students would evacuate.
Student Zach Lawford told KHOU he heard the first shot "clearly" and heard as many as 20 more shots.
Rabon described several minutes of chaos with students crying before they reminded each other to use active shooter training techniques they had learned in drills several times already that year. They barricaded the door, shut off the lights and sheltered in place.
"It was just so scary, you could hear him walking, you'd hear him walking right past our classroom," Rabon said. "You could hear the shots, 'boom, boom, boom.' Thank God he didn't come into our classroom."
Rabon said he believed the shooter to be a former student, either a sophomore or a junior. He described him as a quiet and "weird" type of guy who stood out because he wore a trench coat every day.
Tyler, a senior who was inside the school, told KHOU his friend saw someone walking down the hallway with a gun. Someone pulled a fire alarm, and as students ran outside he heard three shots. Once outside, he said he heard four more shots, ran to the treeline for cover and jumped a fence to flee.
He also said he saw a girl who had been apparently been shot in the kneecap, and had a bandage around her knee and was limping. He said firefighters came to assist her.
When asked what was going through his mind, he said, "Living, stressing, trying to get away....Now I'm worried about everyone else."
Lauren Little was in math class on the opposite side of the school when they heard the fire alarm. Her teacher said there wasn't a drill scheduled and she immediately began ushering them out of the building, Little told KHOU.
She said they crossed a highway and took cover behind a car wash with "people crying and screaming everywhere."
"One minute we're sitting there doing math problems and then we're being told to run for our lives," Little said.