Now I thought the same thing about my state due to the signs on the building but, after speaking with a police officer who happened to be quite knowledgable in the area he stated that A) there is no case law on the subject B) the law says only those that are "licensed" to carry a weapon may do so on school property and C) if he were to come across someone who is legally licensed to CCW on school property he would not sound any alarms.
That's the way it ought to be.
I just looked into it a little further and the Revised Code of Washington states that if you are convicted of carrying a firearm onto school grounds you will lose your CCW permit for three years. Also, anyone between the ages of 12 and 21 caught carrying a firearm onto school grounds SHALL be held in confinement in juvie or adult jail for 72 hours, and SHALL NOT be released until examined by a "mental health professional". The libs believe that if you have a firearm and bring it onto school grounds you must be a loon.