i don't believe ATS would stand for "Air Training School" as cadet pilots in Canadaduring WWII progressed from EFTS (Elementary Flight Training School) through SFTS (Service Flying Training School to an OTU (Operational Training Unit). Those destined to become mechanics and technicians attended TTS (Technical Training School) such as the one at St. Thomas, Ontario while RCAF clerks and drivers attended No.1 CTS (Composite Training School) at Trenton. Ontario. There was also No.1 ITS (Initial Training School) in Toronto, Ontario where cadet pilots learned drill, rank structure, dress, and deportment. I've scoured my books on the history of the RCAF and no reference to ATS found.