The shooting occurred at Freeman High School in the tiny town of Rockford, about 26 miles south of Spokane near the Idaho border.
Three other students were injured before a school janitor was able to subdue Sharpe, who is also a 15-year-old student at the school.
The school board has also approved adding additional security cameras throughout the inside and outside of school district building.
At Spokane Public Schools, resource officers work on developing relationships with students.
The suspect was detained; the Spokane County Sheriff's Office tweeted. He rode the bus to school with them stashed in his duffel bag.
Michael Harper, 15, a sophomore at the school, told The Associated Press that the suspect was a classmate who had always been obsessed with past school shootings.
As the suspect's name circulated online and in local press, the Spokesman-Review reported that he had a well-known history of firing guns on his YouTube channel.
The day before the school shooting outside Spokane, the rural school district conducted a routine lockdown drill.
"That student came to school armed", Knezovich said.
CNN reported that three other juveniles were in stable condition at a hospital.
An "active shooter" was arrested last Friday at a school in Columbus, Ohio, although no one was injured in that incident. "I thought he was a lot wiser (after his father died)".
"I was putting my backpack away and I heard a loud pop", Elisa Vigil, a freshman, told the newspaper.
The shooter had many friends and wasn't bullied, Harper said, calling him "nice and amusing and weird" and a huge fan of the TV show Breaking Bad.
"Knowing the type of person that his father was, it doesn't surprise me one bit that Sam tried to stop the shooter", she said.
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.
"It's been unbelievable, within probably 15-20 minutes of hearing about it, I'd already received a phone call, I'd already received a text message saying that their school is OK", she said. You never think about something happening like this at a small school.. She commended the school district for its communication with parents.
We are profoundly saddened by the violence at Freeman High School and offer our deepest sympathies to the victims, their families and all members of the Freeman Schools community, Maben said.
Washington Gov. Jay Inslee issued a statement shortly after the shooting.