You are not allowed to enter a building with a firearm. The RCW's state it clear as day.
(6) Except as provided in subsection (3)(b), (c), (f), and (h) of this section, firearms are not permitted in a public or private school building.
(b) Any person engaged in military, law enforcement, or school district security activities. However, a person who is not a commissioned law enforcement officer and who provides school security services under the direction of a school administrator may not possess a device listed in subsection (1)(f) of this section unless he or she has successfully completed training in the use of such devices that is equivalent to the training received by commissioned law enforcement officers;
(c) Any person who is involved in a convention, showing, demonstration, lecture, or firearms safety course authorized by school authorities in which the firearms of collectors or instructors are handled or displayed;
(f) Any nonstudent at least eighteen years of age legally in possession of a firearm or dangerous weapon that is secured within an attended vehicle or concealed from view within a locked unattended vehicle while conducting legitimate business at the school;
(h) Any law enforcement officer of the federal, state, or local government agency.