Perhaps you were a passenger in a rental vehicle. California law allows passengers to ride in taxis, limousines, etc. Unless you are the driver, you can challenge the citation by showing that you were in a rental vehicle. While you can drink at your own private party on the bus, California law states that anyone under age traveling on a party bus that contains alcohol must be accompanied by a companion who is at least 25 years old.
Before renting a graduation limousine, party bus, or other type of luxury transportation for the prom, the limousine provider must ask underage guests and their parents to sign a contract for minors and alcoholic beverages. In fact, before you rent a graduation limousine in San Francisco or anywhere else this upcoming graduation season, be sure to choose a limousine service that requires this contract.