This question is never directly answered in the first Chapter, rather we have to glean what clues we can from the conversation of Piggy and Ralph. It is clear that a group of English boys were being taken somewhere in a plane, but Ralph and Piggy disagree about what they can remember:
The fair boy allowed his feet to come down and sat on the steamy earth.
"He must have flown off after he dropped us. he couldn't land here. Not in a place with wheels."
"We was attacked!"
"He'll be back all right."
The fat boy shook his head.
"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."
It is clear that the crashing plane has made the "scar" in the land that is referred to, but now a storm has dragged it out to sea. Later reference is made to an atom bomb, which has wiped out an airport, and causes Piggy to say "they are all dead." It is clear then that this novel is set in a time of war.
Well so far that i know is that they are british boys students and it was during WW1 and on they way to Australia and were bombed