Douglas County Sheriff Tony Spurlock said shots were fired in two separate locations in the high school section of the STEM School of Highland Ranch, which serves grades kindergarten through 12th.
Before officers arrived, one heroic student, Brendan Bialy, tackled one of the gunmen.
Bialy's father told the New York Times that 'two students entered the classroom and one pulled a gun out of a guitar case'.
He said 'his son and two friends tried to tackle the gunman but one of the boys was shot in the chest', according to the Times.
That account was corroborated by the father of one of the teenagers who was shot.
Fernando Montoya told The Denver Channel his son, who was shot three times but is expected to survive, told him a shooter produced a gun from a guitar case.