After finding the gun and more than 400 rounds of ammunition, Frontier Middle School and the adjacent Pioneer Elementary School were locked down Wednesday for two hours, The Columbian reported.
Officers said they also found several knives.
No injuries were reported.
The boy was booked Wednesday into the Clark County Juvenile Detention Center, The Columbian said.
The Evergreen Public Schools' Facebook page was filled with complaints from parents, most expressing concerned they weren't notified of the lockdown.
"As a parent, any incident that involves my children even in the vicinity of a weapon constitutes an emergency and I have the right to be informed," Tara Nolan Monroe commented. "Any parent who is involved in and concerned about their children would not want to learn of a situation like this after the fact, particularly from a public institution."
Parents were alerted about the incident through the school's and district's Facebook pages and the district's website, an Evergreen Public Schools representative said.
Middle school students had a letter explaining what happened when they were dismissed, but elementary school students did not get a letter because they were on an early release day, the representative said.