All three victims of the shooting outside Rosemary Anderson High School in north Portland, two boys and one girl, were "conscious and breathing" as they were rushed to a local hospital, the Portland Police Bureau said in a tweet.
A spokeswoman for Legacy Emanuel Medical Center confirmed that the hospital was treating three patients from the incident but declined to give out any information on their condition, citing medical privacy laws.
It was not immediately clear if the victims were students at Rosemary Anderson, but police said they ran to the school following the shooting, shortly after noon, and that it was immediately placed on lockdown.
The shooter ran from the area, police said, and officers were searching the suspect in nearby neighborhoods using dogs.
"Officers have cleared the school. Area is safe and secure," the police bureau said on Twitter, adding that it was not considered an "active shooter situation."
The Portland Oregonian newspaper reported that a 17-year-old was shot in the back and that a female student was struck in the chest. There was no immediate information on the third victim.
The paper said that FBI agents were on the scene to assist local police and that investigators believed the suspect may have ties to a local street gang.
Aly Wright, who works at the Coffeehouse Five coffeehouse near the school, said her customers heard several gunshots, followed by the quick arrival of police.
"This whole block is on lockdown," she said.
Jefferson High School and Portland Community College were also both placed on lockdown following the shooting and traffic was being diverted from the area, police said. Parents were directed to go to a staging area several blocks from the school
According to its website, Rosemary Anderson is an alternative high school in north Portland serving up to 190 "at-risk" students, many of whom are homeless or who had been expelled or dropped out of public high schools.