She, too, heard the shots. But it wasn't until she heard the sirens wail that she and her classmates put it together: A shooting at the school.

"Everybody was crying," she told CNN.

Alexis and her classmates huddled together in the back of the classroom.

"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 later told ABC's Sawyer.

"Kids were crying, not really like screaming, but they were all huddling together."

Back in Vollmer's classroom, she was concentrating on her job "to keep them safe."

There was no announcement over the school's loudspeaker to announce an emergency, a routine occurrence during emergency drills.

"My instinct was that it wasn't good," Vollmer said.

What Vollmer didn't know was that the principal, Dawn Hochsprung, and school psychologist Mary Sherlach had been shot and killed.

Soon police officers were banging on Vollmer's classroom door.

The kindergartners were told to line up and cover their eyes as they were led by police out of the school to a nearby firehouse, Vollmer said.

Police also evacuated Roig and her students to the firehouse.

Somewhere in the school building, authorities later said, was the gunman's body. Police believe he committed suicide, turning a gun on himself.

Also in the building, the bodies of classmates and faculty.

As reports of the shooting made their way around town, frantic parents descended on a nearby firehouse where the children had been taken.

"Why? Why?" one woman wailed as she walked up a wooded roadway leading from the school.

Inside the firehouse, Vollmer's kindergartners were beginning to understand something terrible had happened.

"They saw other people upset," Vollmer said. "We just held them close until their parents came."

Alexis also ended up at the firehouse. There, she was reunited with her mother.

"It just doesn't seem real," Alexis' mother told WABC. "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."