Parkrose School District Superintendent Michael Lopes Serrao said in a letter to families Friday evening that before the incident two students had informed a staff member of "concerning behaviour" by that student.
Security staff then responded, found the student and quickly disarmed him, he said.
"Thanks to their heroic efforts all students and staff are safe," Lopes Serrao wrote.
Yakots, the police spokesman, said the first responding officers found the gunman being detained by the staff member in the hallway. A firearm was recovered at the school, he added.
Parkrose was evacuated and a nearby middle school was on lockdown for several hours as the investigation unfolded. The high school students were bused to a nearby parking lot where they were reunited with their parents.
The outcome was "the best-case scenario, absolutely," Yakots said. "The staff member did an excellent job by all accounts, (and) our officers arrived within minutes and went right in."
Students recounted how the student entered their government class in the school's fine arts building — separate from the main building — just before noon. Lowe had been in the classroom earlier looking for the student, whom other students identified as an 18-year-old senior at Parkrose.
About 10 minutes before the end of class, the student appeared in the doorway in a black trench coat and pulled out a long gun from beneath his coat, senior Justyn Wilcox, who also was in the room, told The Oregonian/OregonLive.
The student didn't point the gun at anyone, Alexa Pope said.
Students fled out the back door because the gunman was blocking the main doorway.
"As I was running, I was just like, Lord don't let this be it," Pope told the newspaper.
In college, Lowe was a star wide receiver at the University of Oregon, playing from 2011 to 2014. He caught 10 touchdown passes in his college career and had nearly 900 receiving yards. He also saw playing time on special teams.
After college, he worked as an offensive analyst for the San Francisco 49ers and as an analyst for the Philadelphia Eagles.
Lowe began working at Parkrose last year as the school's head football and track and field coach, according to his LinkedIn profile. Before that, he worked for his high school alma mater, Jesuit High, where he earned state defensive player of the year as a defensive back and was a standout sprinter.
Students, parents and co-workers lauded Lowe's actions on Twitter, calling him a hero. Several also noted that the school's prom, scheduled for Saturday, was still on.
Olivia Katbi Smith wrote on Twitter that she coaches track with Lowe at Parkrose and wasn't surprised to learn what he did."
As soon as I heard what happened I knew it was him because he would do anything for these kids. It was surreal to be waiting for my athletes behind caution tape today, but might have been much worse if not for Keanon," she wrote.
Lopes Serrao said the student with the gun will not be returning to school and that school will resume on Monday as usual with an enhanced security presence.
