Lt. Roy Butler of the Battle Ground Police Department said the teenager had no explosives, The (Portland) Oregonian reported. His computer was seized for examination.
The boy was released to his parents after the arrest. His name has not been released.
The threat was made on the Battle Ground schools website against one of the boy's teachers at Chief Umtuch Middle School, police said. The school shares a campus with Battle Ground High School and three other schools, and all were closed Wednesday.
They are expected to reopen Thursday.
Ron Burgundy interviews Peyton Manning on SportsCenter
Toddler uninjured after being knocked over by Obama family dog