Why, In Lord Of The Flies, are the boys on the island?

One can infer from the text that the boys were a group of British schoolchildren. They had been evacuated from their country during a devastating atomic war. The evacuation seems to have been an emergency procedure since it was clear that they had been taken directly from their schools because they were all still dressed in their uniforms, or as with Jack and his choirboys, in their choirboy attire, as suggested by the following extracts from chapter one:

Though he had taken off his school sweater and trailed it now from one hand, his grey shirt stuck to him and his hair was plastered to his forehead. 

Some were naked and carrying their clothes; others half-naked, or more or less dressed, in school uniforms, grey, blue, fawn, jacketed, or jerseyed. There were badges, mottoes even, stripes of color in stockings and pullovers.

Shorts, shirts, and different garments they carried in their hands; but each boy wore a square black cap with a silver badge on it. Their bodies, from throat to ankle, were hidden by black cloaks which bore a long silver cross on the left breast and each neck was finished off with a ham-bone frill.

The evacuation itself seems to have gone smoothly but the plane developed problems because it had been hit during an attack whilst flying to a safe haven, as suggested by 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.”

The pilot was forced to make a crash-landing on the island, believed to be located somewhere in the Pacific, as suggested by Jack's references when he speaks of Simon:

“He’s always throwing a faint,” said Merridew. “He did in Gib.; and Addis; and at matins over the precentor.”

'Gib.' is a clear reference to Gibraltar and 'Addis' refers to Addis Ababa, whilst 'matins over the precentor' was Jack's attempt at humor, obviously only understood by the choirboys who sniggered at his remark.

The plane skidded onto the island, leaving a huge scar. It was later dragged into the sea in a massive storm to then disappear forever.

There was a suggestion by Piggy that some boys could still have been on the plane when it was dragged out to sea.

“That storm dragged it out to sea. It wasn’t half dangerous with all them tree trunks falling. There must have been some kids still in it.”

The boys were stranded on the island without any adult supervision and they had to find a way to not only take care of themselves but also figure out how to get rescued.