Ashlyn Gosa, a senior at Apalachee High School in Georgia, was in class when she said she heard multiple rounds go off outside her door.
"Everyone in my class knew exactly what it was and everyone immediately dropped to the ground," the 17-year-old told ABC News.
Gosa said she hid under her teacher's desk, which was in direct view of the classroom door.
MORE: Georgia high school shooting suspect and his dad make 1st court appearancesThe shooting suspect "came to our door and just, like, banging frantically on the door, shaking the door handle trying to get in," she said. "I'd never seen him before, but I knew he wasn't in our class, so I knew it wasn't a student trying to get back in."
Gosa said her mom called her when she was under the desk. The teen answered, saying she whispered, "Mom, there's a shooter."
"I told her I was alive," Gosa said. "And I was just scared that if that door or that lock gave in at any second, that it probably would've been the end for all of us."
MORE: What to know about gun laws in Georgia after the Apalachee High School shootingThe suspect then moved on from their room, Gosa said, and the teacher pushed a filing cabinet in front of their door as a barricade.
"SWAT came and got us. We were told to walk out into the hallway with our hands up," Gosa said.
While being evacuated, the teenager said she saw a victim on the ground surrounded by "a huge pool of blood."
MORE: Apalachee teacher fatally shot near doorway to his classroom: 'He was trying to crawl back to us'Two teachers and two students were killed in Wednesday morning's shooting in Winder. Nine people were injured.
Two days later, Gosa said she still feels numb and "can hear the gunshots plain as day."
"You never think it'll be your school until it is," she said.
"The fact that it happened just makes you feel really unsafe," she added.
The 14-year-old suspect, student Colt Gray, is charged with four counts of felony murder. More charges will be filed, prosecutors said.
The teen's father, Colin Gray, was arrested Thursday and charged with four counts of involuntary manslaughter, two counts of second-degree murder and eight counts of cruelty to children, the GBI said. Colin Gray is accused of "knowingly allowing his son, Colt, to possess a weapon," GBI Director Chris Hosey said.