Luke's Military Training

FWIW;
The Rebel forces were not much on protocol.
Luke might have been "conscripted" then moved up the ranks quickly.
As a kid, I had a Star Wars children's book that said Luke Skywalker did go thru a formal military training program, academy.
I'd add in the 1980s era NPR(National Public Radio) version of The Empire Strikes Back, the convoy commander for Echo Base was killed by a stray asteroid as his X wing came out of hyper-space . This put Luke in line for "Commander" of the fighter wing.

 

Luke was hired because he was already a decent pilot who could shoot womp rats two meters long. Besides, the Rebel Alliance was desperate for anybody at that point in the film.

 

Well, Yavin was evacuated immediately after the ceremony, as shown in the Princess Leia comic. Rebels likely brought in other transports and cells to help evacuate equipment.

We'll also see more of the Yavin base in Rogue One! Possibility that U-Wing pilots also wore similar flight suits. Otherwise, we'll have to figure out what George was thinking - if he even remembers correctly or not.

 

The real reason why Luke was sent into battle with no training is because of a deleted scene in the Yavin IV hangar. The deleted scene features Luke, Red leader and Biggs Darklighter. Red Leader tells Luke that he once met Lukes father in the Clone Wars (not the modern interpretation of it) and hoped if Luke had half of his fathers skills, he would do all right.

Here is the full original scene:

^ This entire scene was not used in the theatrical 1977 version. Bits of this scene were later cut together for the 1997 Special Edition version and we get the Luke meets Biggs scene out of it. The cuts and edit is clear in the video below at 0:16 seconds, someone is edited walking in to mask the joins in the editing process where talk of Lukes father had taken place.