The shooting took place in suburban Denver school, and several are in critical condition. The victims are all 15 and older.
Authorities said two suspects were in custody following the shooting.
A Douglas County Sheriff's Department spokeswoman told reporters that there could be a third suspect in the school, which was still being searched.
Lines of firetrucks, ambulances and law enforcement vehicles were at the scene, and medical helicopters were deployed.
The shooting occurred at STEM School Highlands Ranch, a public charter school with more than 1,850 students in kindergarten through 12th grades.
It's near a sheriff's department substation, and authorities responded quickly and in force to the shooting.
Douglas County Sheriff Tony Spurlock said at a news conference Tuesday that two shooters walked into the school and opened fire on students in two classrooms.
He said a number of students were shot and wounded.
The sheriff says the shooters are in custody, believed to be a man and a minor boy.
Spurlock says at least a handgun was recovered, but he didn't release other information on weapons
(Inputs from agencies)