In Lord of the Flies, how did the choir boys, Ralph, and the other boys get separated? Why did the choir boys not look like they had just been in a plane crash?

Golding does not spend a great deal of time describing the actual events of the plane crash, so it is difficult to know exactly what happened prior to the boys coming out of the jungle for the first meeting.
It is clear that the plane broke into pieces, according to the conversation between Ralph and Piggy. It is also clear that at least one piece of it hit the island and made the "scar" in the jungle. During the crash, it is also clear that the boys were separated, either thrown out of different parts of the plane or perhaps they simply scattered when the plane hit the ground.
The choir boys somehow managed to save their "togs" and Jack forces them to put these on before they march to the meeting in formation. The uniformity allows them to appear as though they'd just come from a performance rather than coming out of the jungle following a plane crash.