CG - if you can get your hands on a copy, Denis Winter's The First of the Few is pretty good about the commissioning/training aspects that were in place at about the time Aces High is set.
Procedure for commissioning was a bit lax in 1914, though. Maurice Baring, the journalist and author was a good chum of Sir David Henderson (GOC RFC) and wanted to 'do his bit' even though he was 40 years old.
He went and saw Sir David, who promptly commissioned him (to be his MA). Henderson sent Baring off to Gieves & Hawkes to get kitted out, along with a couple of his SO3s (who knew what Baring had to buy).
When it came to putting the rank onto the uniform, they realised that Henderson hadn't said what rank Baring should hold. The SO3s conlcuded he was a bit old to be a 2nd Lieutenant, so told the tailor to sew on the pips for a Lieutenant...
And then we get Jack Slessor. Slessor wanted to join the RFC, and it was discovered that a close friend of his uncle (an army officer) was the chap in charge of selecting officer candidates. Slessor's aptitude test was - I kid you not - to walk across the room in a straight line. He passed, and was told to obtain the signatures of his father (a retired Colonel recalled to the colours) and his former headmaster (Haileybury), then attend a medical board.
The medical board declared Slessor "totally unfit for any form of military service", but his Uncle's friend managed to lose the paperwork and instead sent Slessor orders to report to Brooklands to learn how to fly. Slessor later recorded "I do not hold that out as the ideal method of the selection of officers" - but I guess that was a relatively easy thing to say when writing that as MRAF Sir John Slessor GCB, DSO, MC....