I did find a WAC that states:
(8) No person or group may use or enter onto college facilities having in their possession firearms, even if licensed to do so, except commissioned police officers as prescribed by law.
If you read the whole chapter, it refers to outside organizations and groups using school facilities. Would it apply to regular students engaging in regular school activities?
Edit to add: Doesn't this WAC go against RCW 9.41.290 in regards to state preemption? Just like Seattle's ban on firearms in parks. That one got revoked because it was in violation of state preemption.