Lowe -- one of Marcus Mariota's favorite targets back at Oregon -- reportedly tackled a man with a gun near the tennis courts at Parkrose High School in Northeast Portland.

Unclear what the man's intentions were -- cops have yet to say his motive -- but Oregon Live reports, "a parent said students reported that the suspect, a fellow student, had brought a gun to the school with the intention of committing suicide."

Cops say they've taken the suspect into custody ... and school officials now say all students are safe.

Keanon -- who's reportedly the high school's football coach as well its track and field coach -- was a star with the Ducks, tallying 891 receiving yards and 11 TDs in 68 career games.