Police gave no information on the weapon used, or whether the shooter was one of Freeman High School's 327 students, but two students told KXLY television that they knew him.
"He was handing out a note to his friend saying he would do something stupid that could get him killed or in jail," one said. "That was a few weeks ago."
Schaeffer said he did not know if the suspect was a student at the school or what may have motivated the gun violence. None of the wounded victims were identified.
The fire chief described a chaotic scene at the school, with the sounds of bullets echoing through the halls prompting fears that there was more than one shooter.
Local television stations showed the school surrounded by police and fire vehicles, parents running the scene. Some parents got out of their cars and walked up to a mile rather than wait in traffic, KHQ-TV reported.
The high school has 327 students, according to U.S. News.
Spokane Public Schools on Twitter said due to a reported shooting near Freeman High School, all schools "are in full lockdown as a precautionary measure."
The Spokane Sheriff office is "going room 2 room, floor by floor" in the school. They further tweeted asking people to avoid the area.
#Frreman School Shooting-MiddleSchool & Elementary on lock down. Multiple DeputiesOfficers/Medical @ scene. Safety of students #1 PRIORITY— SpokaneSheriffOffice (@SpokaneSheriff) September 13, 2017
A Twitter user named Christina identified herself as a junior at the high school and said she had been evacuated following at least four shots. She tweeted a picture of anxious-looking students sitting on the floor in a classroom.
"This morning's shooting at Freeman High School is heartbreaking. All Washingtonians are thinking of the victims and their families," Governor Jay Inslee said on Twitter.
Spokane placed all schools in the district on lockdown at about 10:30 a.m. PDT (1730 GMT), following the shooting, but an hour later said on Twitter that it had been lifted.
The United States has had an average of 52 school shooting incidents a year since a gunman killed 26 young children and educators in Newtown, Connecticut, in 2012, according to Everytown for Gun Safety, a gun-control group founded in response to that massacre.
With inputs from Reuters and AFP