Police recovered the weapons and ammunition earlier in the day at Frontier Middle School. No one was hurt.
The boy was interviewed by detectives and arrested for investigation of one count of attempted murder, police spokeswoman Kim Kapp said in a statement late Wednesday night. The boy, who was not identified, was booked into the Clark County juvenile detention center, she said.
Officers responded to the school about 9:30 a.m. Wednesday to investigate a report that ammunition had been found there. Police located the ammunition and multiple weapons.
The middle school and adjacent Pioneer Elementary School were locked down for about two hours.
The police major crimes team continues to investigate, Kapp said.
The spokeswoman was not immediately reachable by phone or email to answer additional questions.
An estimated 900 students attend the middle school and about 600 attend the elementary school, The Columbian reported.
Parents were primarily alerted about the ammunition and weapons through the Frontier and district Facebook pages, along with the district website, said Kris Fay, Evergreen Public Schools spokesman.
While middle school students were sent home with an explanatory letter, elementary school students did not get a letter because they were on an early release schedule, Fay said.