California Code, Education Code - EDC § 39842

(a) Any person who enters a schoolbus or school pupil activity bus without prior authorization of the driver or other school official with intent to commit any crime and who refuses to disembark after being ordered to do so by the driver or other school official is guilty of a misdemeanor and is punishable by imprisonment in the county jail for not more than six months, by a fine of not more than one thousand dollars ($1,000), or by both.

(b) A school district or county superintendent of schools may place a notice at the entrance of a schoolbus or school pupil activity bus that complies with the requirements of paragraph (3) of subdivision (c) of Section 1256.5 of Title 13 of the California Code of Regulations and that warns against unauthorized entry.