"We still do not know if there are more shooters in the school or more injured", Douglas County Undersheriff Holly Nicholson-Kluth told reporters.
Nicholson-Kluth said police and deputies got there nearly immediately and heard shots as they arrived.
The STEM School has about 1,800 students, the undersherrif said. The nature of their injuries is unclear.
FBI and ATF agents were also responding to the school, federal investigators confirmed.
The sheriff's office subsequently told parents to go to the Northridge Rec Center to pick up their children.
Trump goes on retweeting tear as he continues to attack Biden
Jill Biden, the former vice president ripped into what he believed are the cultural consequences of a Trump presidency. More than half of all Democrats said they would be less likely to support a candidate over 70 years old.
It was unclear whether there were any fatalities, she said in a brief media briefing.
Paulson's son, Christian, said he was in study hall at the school when he saw "a bunch of kids running out and saying 'School shooter, school shooter!'"
A tweet sent by the Sheriff's Office at 4:40 p.m. EDT describes the area as an active and unstable scene. The school is located at 8773 South Ridgeline Blvd., Highlands Ranch.
"As officers were arriving at the school they could still hear gunshots", Nicholson-Kluth said.
More information wasn't immediately available.
This is a developing story.
- Pressure grows on Attorney General Barr over Mueller
- US joblessness at 3.6%, lowest since 1969
- Nadal battles past Struff in Barcelona
- Google's struggling Pixel sales invite comparison with Microsoft's smartphone failures
- Android Auto gets a new look, faster access, and a dark theme
- Vh1 India to air 2019 Billboard Music Awards on 2 May
- Palestinian Baby Dead, Pregnant Mother Hurt In Israeli Strike
- Thousands affected as Scandinavian Airlines strike continues
- Chelsea to employ makeshift defence in Frankfurt
- Positive Results for Arsenal and Chelsea in Europa League