A third grade student said the kids were so scared she thought she was going to throw up. Another said he hid in a closet.
The gunfire erupted during first grade teacher Kaitlin Roig's morning meeting with her 14 students, what she called "a happy, amazing part of the day."
That day quickly turned into a nightmare.
"Suddenly, I heard rapid fire... like an assault weapon. I knew something was wrong," Roig, 29, told "World News" anchor Diane Sawyer.
"It was horrific," she said. "I didn't think we were going to live."
Alexis Wasik, an 8-year-old third grader, was startled when she heard someone rapidly firing off rounds inside another classroom. At first, she didn't know what was going on, but then she began to hear the sirens wail.
"We heard an ambulance and police officer come and everyone was a little scared crying and I felt actually a little sick and like I was going to throw up," she said. "Kids were crying, not really like screaming, but they were all huddling together. They felt so sick."
It was 9:41 a.m. when the first 911 call came into Connecticut State Police that multiple students at Sandy Hook Elementary School were locked in a classroom with a gunman. Adam Lanza killed 20 students and six adults at the school.
When the shooting began, Roig said she quickly got up and closed her classroom door and ushered the children, all aged 6 and 7, into the class bathroom. She helped some climb onto the toilet so they could all fit. Roig said she then pushed a wheeled storage unit in front of the door.
"We all got in there. I locked us in," she said. "I don't know if [the gunman] came in the room... I just told them we have to be absolutely quiet."
"If they started crying, I would take their face and tell them, 'It's going to be OK,'" Roig continued. "I wanted that to be the last thing they heard, not the gunfire in the hall."
Roig said she just tried to stay strong for her students, but she didn't think they would make it out of the classroom alive.
"I thought we were all going to die," she said through tears. "I told the kids I love them and I was so happy they were my students... I said anyone who believed in the power of the prayer, we need to pray and those who don't believe in prayer" think happy thoughts.
Throughout the ordeal, Roig said her students were being very good and she tried to remain positive for them.
"They asked, 'Can we go see if anyone is out there... I just want Christmas... I don't want to die, I just want to have Christmas," she said.
The gunfire didn't last very long, Roig said, but even when it stopped, she refused to take the kids out of the bathroom. When she heard knocking on the door a little while later, she said heard voices saying they were police officers, but she refused to open the door. Scared it was the gunman trying to lure them out, Roig told them to slide their badges under the bathroom door to prove their identities.
"I didn't believe them," she said. "I told them if they were cops, they could get the key... They did and then unlocked the bathroom."
Students who spoke to ABC News Radio told more stories of teachers who protected them during the shooting rampage. One 9-year-old boy said his class heard "a lot of bangs" and at first they thought a custodian had "knocked stuff down." Then they heard screaming.
"Police came in, said like 'Is he in here?' Then he ran out and then our teacher, somebody, yelled, 'get to a safe place.' So we went to the closet in the gym," the boy said. "The police were like knocking on the door and they're like 'we're evacuating people, we're evacuating people,' so we ran out."
After the police got Roig and her class out of their room, she said she and the children were taken to the nearby fire station, which had been set up as a staging area for parents to come pick up their kids.
That fire house, where Christmas wreaths and poinsettias are for sale, has been turned into a place of grief where frantic parents were either reuniting with their children or learning that their children are dead, or were still waiting for word.
Children stared wide-eyed as they watched state police troopers in body armor, holding raised rifles, quickly rush to secure the scene at their school. Parents said they had never been so panicked. One father, hoping to preserve a semblance of innocence, shielded his son's eyes with his forearm.
Wasik's mother said she found out about a shooting through the school's alert system, which sent her a message about a lockdown, and is still in disbelief.
"It just doesn't seem real," she said. "It feels like a nightmare. You drop your kids at school, hugs and kisses, have a good day, I'll see you later and see you at the end of the day and you never know.. in 20 minutes from now what's going to happen. And you count your blessing everyday for what you have."
ABC's Dan Harris, Aaron Katersky and Marcus Solis contributed to this report.