Student Matthew Walker told ABC News the gunman fired through their door.
"He was shooting through the glass in the door ... It was really bad," Matthew said. "There were bullet holes everywhere. Everyone was freaking out. I did not see the shooter, but he did shoot through my class. He couldn't get inside because the door was locked."
Another student initially thought the sound of gunshots was a garbage truck, he told ABC News.
"It was really loud. Then I heard it again," Michael Katz said. "Then I heard frantic screaming. I've never heard such loud screams in my life. Then we realized there was a shooting at my school."
Katz said he and his classmates hunkered down until the gunfire subsided four to five minutes later. He heard police enter the hall and knock on the door before they led the students out of the classroom.
"We moved in a single line, then lying around the wall from my room there were two dead bodies on the ground," Katz said. "There was a boy and a girl. They were both extremely pale and bleeding."
Nicole Baltzer, an 18-year-old student, was in trigonometry class when the fire alarm went off. As students filed out, she told CNN that's when bullets started to fly.
"I heard so many gunshots, at least like six. They were very close," Baltzer said. As a police officer led Baltzer and her classmates outside, they passed a classroom. The officer reportedly told her to close her eyes, "there's nothing good to see in there."
The Associated Press, ABC News, and CNN contributed to this report.