Authorities have tentatively identified the gunman but weren't ready to release the name, police Chief Scott Anderson said.
They were in the process of notifying the family of the gunman and student who was killed.
Anderson said the teacher's injuries weren't life threatening, and he was treated at the scene. He was identified as physical education instructor Todd Rispler.
During an evacuation of the school, another gun was found and one person was taken into custody. Anderson said that weapon and arrest were not related to the shooting.
The attack panicked students at the school after a lockdown was ordered and they were told to go quietly to their classrooms.
Freshman Morgan Rose, 15, said she hunkered down in a locker room with another student and two teachers.
"It was scary in the moment. Now knowing everything's OK, I'm better," she said.
Freshman Daniel DeLong, 15, said after the shooting that he saw a physical education teacher at the school with a bloodied shirt. He said he was texting friends to make sure they were all OK.
"It just, like, happened so fast, you know?" he said.
Anderson said he was sorry for the family of the slain student.
Anderson said two on-campus police officers were the first to respond to reports of a shooter. He said they and a tactical team sent to the school "brought this to a conclusion." Anderson did not say how the shooter died.
"Today is a very tragic day for the city of Troutdale," the chief said.
Gov. John Kitzhaber added in a statement: "Oregon hurts as we try to make sense of a senseless act of violence."
The first reports of shots fired came at 8 a.m. on the next-to-last-day of classes. Police initially seemed uncertain about whether there was a live shooter in the school.
Students were eventually led from the school with hands up or on their heads. Parents and students were reunited in a supermarket parking lot.
Mandy Johnson said her daughter called from a friend's phone.
"I thank God that she's safe," said Johnson, who has three younger children. "I don't want to send my kids to school anymore."
The Reynolds School District issued a statement mourning "the loss of one of our students."
The Oregon violence came less than a week after a gunman opened fire on a college campus in neighboring Washington state, killing a 19-year-old man and wounding two others. It follows a string of mass shootings that have disturbed the nation, including one on Sunday in Nevada that left two Las Vegas police officers and a civilian dead.
The Tuesday shooting was the first fatal school shooting in Oregon since May 1998 when 15-yeatr-old Kip Kinkel killed two students and wounded 25 others at Thurston High School in Springfield near Eugene. He killed his parents prior to the attack and is serving a 111-year prison sentence.
Reynolds is the second-largest high school in Oregon, with about 2,800 students. Its students come from several communities.
AP writers Gosia Wozniacka in Portland and Alina Hartounian in Phoenix contributed to this report.