The boys are traveling from England on an airplane. There is reference to a man with a megaphone. It feels as if they are trying to escape something in England. There is talk of the atomic bomb among the boys. Once they land on the island however, the man with the megaphone dies. Thus, only the boys are left.
This has been a problem for many readers and critics. The exact circumstances aren't very plausible. The boys are being evacuated because there's a war going on, and their plane is attacked. But how could the entire plane just disappear? How do the boys get off a burning plane without parachutes or something? Why are there no dead or wounded? It's all pretty far-fetched.
The answer is that you mustn't take the situation literally. Golding is writing a fable, and as such he is at liberty to tweak reality as much as necessary to get his theme across. The important thing to remember is not how they got there, but that the fact that the boys are safe, with all their physical needs provided-for, removed from the violence of war, and free to let their natural, child-like goodness and innocence to blossom.
With a copyright date of 1954, we can safely assume it's an evacuation during a futuristic World War III. The boys' plane was shot down during that evacuation. There are a couple quotes that clue us into this:
Piggy - "We was attacked." "When we was coming down I looked through one of them windows. I saw the other part of the plane. There were flames coming out of it." "...Didn't you hear what the pilot said? About the atom bomb? They're all dead."
Obviously, their main concern is finding adult leadership - which they don't.
Piggy - "Where's the man with the megaphone?"
This quote is then followed by a short, choppy conversation about the lack of adults on the plane or on the island.