A third person was "tentatively" a third suspect, Douglas County Undersheriff Holly Nicholson-Kluth said.
"Still active and unstable scene".
Though authorities initially said they were looking for a possible third suspect, Nicholson-Kluth said they are no longer looking for another suspect.
"I got a text from a friend who was actually in there", Kelley Paulson, the mother of two students at STEM School Highlands Ranch told Denver 7. Multiple students were injured, according to the sheriff's office.
It's not clear exactly where the shooting happened but law enforcement are believed to be inside the school, searching room to room, according to 9News.
The school is near a sheriff's department substation.
Witnesses said they saw a medical helicopter land on a grassy field.
The sheriff's office directed parents to a recreational center to pick up their children. The school is located at 8773 South Ridgeline Blvd., Highlands Ranch.
The Denver Post reported that all schools in the area where placed on lockdown while police and fire crews responded to the scene.
This is a developing story.