Today, the California Commission on Peace Officers Standards and Training (P.O.S.T.) mandates police agencies conduct formal training for all sworn officers wishing to be P.O.S.T. certified.  It hasn't always been that way.  Prior to WWII, few U.S. police agencies had any formal training at all.  San Diego however was apparently different.
A July 15, 1909, San Diego Union article announced an order to formally train San Diego police officers in police procedure, law and firearms. Issued by Superintendant of Police John Shehon, the order made the San Diego Police Department one of the first in America to mandate formal training.

In 1917, Berkley Police Chief, and renowned police reformer, August Vollmer released his findings in a top to bottom audit of the police department. Among the numererous recommendations was the establishment of a police school.