California Code, Vehicle Code - VEH § 27906

(a) Every schoolbus, while being used for the transportation of school pupils at or below the 12th-grade level shall bear upon the front and rear of the bus a plainly visible sign containing the word “schoolbus” in letters not less than eight inches in height. The letters on schoolbus signs shall be of proportionate width.

Except as provided in subdivision (b), no other vehicle shall display a sign containing the word “schoolbus.”

(b) Notwithstanding subdivision (a), a schoolbus which is also used to transport persons of any age who are developmentally disabled, as defined by the Lanterman Developmental Disabilities Services Act (Division 4.5 (commencing with Section 4500) of the Welfare and Institutions Code), may display a sign containing the word “schoolbus” while transporting those persons to or from vocational, prevocational, or work training centers sponsored by the State Department of Developmental Services.

(c) Every schoolbus, when operated for the transportation of school pupils at or below the 12th-grade level, shall bear upon the rear of the bus, below the rear windows, a plainly visible sign containing the words “Stop When Red Lights Flash” in letters not less than six inches in height. The letters on schoolbus signs shall be of proportionate width.