Update:
According to Highline Public Schools communication director Catherine Carbone Rogers, "(Evergreen) school was on lockdown for about 45 minutes based on a 911 call to police who responded but determined the call wasn’t credible and once they determined that, the school was taken out of lockdown."

The Seattle P.I. com reports that a 15 year old was tracked through his cell phone and arrested in the incident.
http://www.seattlepi.com/local/430971_lockdown30.html

According to the P.I. report the five calls to 9-1-1 came in first at around 9:30 a.m., The claim was made that an AK-47 rifle was in a locker at the school.

20 deputies and detectives responded. A K-9 dog that can sniff out weapons was brought in but no weapons were found.

Three schools in the Highline School District were on lockdown for 45 minutes.

In a precautionary move, Evergreen High School was locked down as well as Cascade Middle School just north of Evergreen and New Start Alternative School.