The OTCs were school cadet organisations - their successor is the Combined Cadet Force. They were mainly in independent and grammar schools, from which it was expected that most officer recruits for the Army would come. They were seen as the junior version of the University OTCs, which still exist.
Officers within each school contingent would be teachers from the school, as is still the case; at the time you are interested in, many would have had war service.
Thinking about it, there's a reasonable chance that your father-in-law gained the swagger stick through being promoted to Cadet Sergeant Major or some other high Cadet rank.
