Why are the boys alone on the island in "Lord of the Flies"?

The boys are alone on the island because their plane crashed.  They were a group of boys from a British Prep. School during a war and their plane was shot down.  They were being evacuated for their safety and crashed in the process of the evacuation.  The pilot – the only adult mentioned as accompanying them on the plane -- died in the plane crash.  There is evidence that there have been other people on the island, for example the parachutist, but at this point when the story is taking place the boys are all alone to fend for themselves.