There have been conflicting reports on the exact number of injured, but three victims were transported to Providence Sacred Heart Medical Center's trauma center, a spokesman from the fire department said.
Their injuries are not known.
Officials said that a suspect was taken into custody after killing one student.
The school is located in Rockford, a town of about 500 people located a few miles south of Spokane, WA.
Schools across the Spokane area were placed on lockdown as a precaution while officers searched Freeman High School room by room. Three other students were hospitalized and in stable condition, according to media reports.
The high school has been cleared; some students have been evacuated to the Freeman Elementary School nearby.
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"He called me and said, 'Mom there are gunshots.' He sounded so scared. You never think about something happening like this at a small school".
One pupil posted a photo of terrified teens taking shelter in a classroom as the gunman rampaged through her school.
"They did a modified lockdown drill yesterday", she said.
No information was immediately available on who opened fire.
And Gov. Jay Inslee's office issued a statement shortly after noon: "All Washingtonians are thinking of the victims and their families, and are grateful for the service of school staff and first responders working to keep our students safe".
PBS NewsHour will update this story as it develops.