(CNN) Children burst into tears in sixth-grader Nate Holley's classroom as gunfire erupted at his school Tuesday.

Two people believed to be students at STEM School Highlands Ranch had opened fire in two classrooms in the suburban Denver school.

"It was really chaotic," the 12-year-old said. "Most of the kids didn't know what to do."

Nate said he froze as gunshots shattered a window. A siren rang and someone in his classroom cracked a joke. His teacher shushed the student and moved them behind a desk and then to the closet.

"I had my hand on a metal baseball bat just in case," Nate told CNN's Brooke Baldwin. "'Cause I was gonna go down fighting if I was gonna go down."

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