how did the boys come to be on this island?

The boys are marooned on the island after a plane crash. There is a distinct group from the beginning; a choir led by Jack Merridew-

'I'm chapter choirister and head boy. I can sing C sharp.'

Ralph - not from the choir group - is elected chief and the tension between the two groups begins here. Piggy, though irritated at Ralph christening him Fatty, votes for Ralph.