The first school "crossing guards" were members of the police force. It is not known of New York City had the first school "crossing guard" in the 1920s. The term doesn't appear that early in the digitized New York Times, but a 1943 obituary (below) indicates that a patrolman had been a "crossing guard" for 14 years.
1 January 1890, Chicago Daily Tribune, pg. 22:
A Railroad Crossing Guard.
5 October 1917, Chicago Daily Tribune, "HEAD OF PAISLEY BANKS LABORS AS CROSSING GUARD," pg. 1:
William H. Paisley, founder and head of the defunct banking firm of W. H. Paisley & WSOns, which operated three private banks de luxe on Chicago's north side, is now a crossing flagman for the Chicago, North Shore and Milwaukee Electric railroad att Oak and Cherry streets, Winnetka, at a salary of $50 a month.
3 September 1931, Los Angeles Times, pg. A1:
SCHOOL WATCH INCREASED
Recruits From Vice and Traffic Divisions of Police
Force Swell Crossing Guard to 180
Making further slashes into the personnel of various police divisions, Deputy Chief Cross yesterday built up the special "School Crossing Watch" to a total strength of 180 men, providing for the policing of four more dangerous school crossings than were covered during the last spring semester.
4 September 1931, Los Angeles Times, pg. A1:
CROSSING GUARDS DRAFTED
Twenty-five Detectives Called to Protect Pupils;
More If Citizens Demand It, Says Cross
22 September 1931, Los Angeles Times, pg. A10:
TO BE ENLARGED
11 May 1934, Chicago Daily Tribune, pg. 31:
28 September 1943, New York Times, Pg. 25:
PATROLMAN BROPHY RITES
600 School Children at Mass for
For fourteen years Patrolman Brophy was assigned to crossing duty at the school.
(Our Lady Queen of Martyrs Roman Catholic Church at Queens Boulevard and Ascan Avenue, Forest Hills - ed.)