Writing in 1954, Golding sets his novel during a fictional atomic conflict, one that could be considered World War III. During the threat of imminent danger to their English homeland, the boys, prior to the start of the novel, had been gathered together for evacuation. This is suggested by the fact that the children respond immediately to the sound of the conch and give Ralph, the one who blows it, "the same simple obedience that they had given to the men with megaphones." The boys were loaded onto an airplane, without very many adults, and the plane left England just in the nick of time. Piggy explains to Ralph what he overheard from the pilot on the ship, pointing to the fact that all the boys' families may well be dead by now: "Didn't you hear what the pilot said? About the atom bomb? They're all dead."
As the novel opens, Ralph is making his way down the "scar" made through the jungle by the crashing plane. Piggy catches up to him and asks, "Where's the man with the megaphone?" It's clear the boys did not know personally any of the adults on board the plane. Ralph is under the mistaken impression that the pilot dropped the children off and then flew away, planning to return shortly. Piggy puts that notion to rest: "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." Piggy explains that the ruined plane was dragged out to sea by the storm during the night. The adults and any children that were still in it would have been dragged out to sea as well and drowned.
The logistics of how the boys got off the plane are unclear. It seems that a man with a megaphone was getting boys off the plane after it crash-landed on the beach. As he went into the wreck for more boys, the storm intensified, enough to fell trees, and dragged the remains of the plane out to sea, drowning any who had not gotten out.
The boys, British schoolchildren, were in a plane that was shot down and crashed on a deserted island. The "shooting down" of the plane is to let the audience know that this is a time of war.
The scene at the very beginning of the book is during one of the world wars. As a bunch of British school children were flying on an airplane in order to escape the warefare their plane was shot down. I believe that it tore the plane in half, sending one part into the ocean and one onto the island since if you read the details only part of the plane was there.