Wis. Stat. § 118.325: Locker searches.

An official, employee or agent of a school or school district may search a pupil’s locker as determined necessary or appropriate without the consent of the pupil, without notifying the pupil and without obtaining a search warrant if the school board has adopted a written policy specifying that the school board retains ownership and possessory control of all pupil lockers and designating the positions of the officials, employees or agents who may conduct searches, and has distributed a copy of the policy to pupils enrolled in the school district.