As long as the school admins follow the legal requirements for creating a WAC then they, as long as it is done correctly, have the authority to enforce those WAC's. They are civil penalties only.
As far as your analogy goes it is fairly flawed. You do not have to be on a public roadway to be arrested for DUI or reckless endangerment.
http://apps.leg.wa.gov/RCW/default.aspx?cite=34.05 is what gives them the authority to create WAC's.