Assume the persona of one of the surviving boys now grown to adulthood, and write the final chapter of his memoirs from his perspective as an adult looking back at his childhood experience on the island. Focus on how this person's understanding of events may have changed with maturity and the passage of time.
Being rescued presents a huge shift of context for the events which take place on the island. I think the boys would realize this rather quickly, seeing that the "savage" group was entirely unnecessary and that the battle with Ralph and Piggy should not have taken on the life-and-death seriousness it did. They only had to wait another couple of days. There was no need to go to war, to see the situation as dire...
As previous posters have said, you must first choose the perspective from which you will write. No Piggy and no Simon, so that leaves Ralph, Jack, Samneric, Roger and Percival if you're interested in a "named" character. If not, of course, you could be any of the boys who simply watched and experienced it all but were not particularly intimate with the major incidents of the story. I might choose Percival, the littlest one there and the one who can barely remember his "civilized" self by the end of the story. Young children are incredibly resilient; however, I think every one of the boys on this island will be forever impacted by this experience. How that will look depends on whom you choose and what you decide to do with him in his adult life.
This topic reminds me of the novel All Quiet on the Western Front by Erich Maria Remarque, a narrative set during World War I in which a young generation of men are lost to the horrors of battle. Remarque wrote that his novel was written to report on a "generation that was destroyed by the war--even when it escaped the shells."
Like the young soldiers of WWI, Ralph, Piggy, Jack, Roger, and the others have been damaged by their experiences and will never be the men that they could have become. Ralph especially seems like the character who might bemoan--to use the words of Bob Seger, songwriter--"I wish I didn't know now what I didn't know then." He probably will seek peace like the main character of Remarque's novel, but never quite find it. Always there will be the torturous memories of his friend Piggy and the wonderful boy named Simon.
As you know, we cannot write your essay for you, but we can help you with how you might attack this assignment.
First of all, you must choose the boy you want to write about. Would it be Ralph? He is the most mature of the boys, and as the novel ends, he is about to be attacked by Jack and his pack of wild animals. The Navy officer arrives just in the nick of time. So perhaps you could write from his perspective - what were his thoughts right before being rescued? What were his fears? What has he learned about human nature? Is he still in contact with Jack? Perhaps you could write about Jack having turned into a criminal, who is now in prison, and Ralph goes to visit him. Ralph could have become a psychologist, and he could go to visit Jack in prison, where he has become a serial killer, and then Ralph would write the final chapter as a flashback - i.e. "I always knew he would wind up this way."
OR...............you could choose Jack. Perhaps as a result of his experience on the island, Jack has a mental breakdown and must spend years in therapy. He could be writing the final chapter as if he were talking to his therapist. Or, Jack could grow up to be an important world leader - perhaps the Prime Minister of England - and he has learned all about how violent mankind can become if not controlled by society's laws. Or Jack could be a lawyer. Or Jack could be a Navy Seal. Or Jack could be an important General. Jack has leadership qualities, but they go wrong on the island. Perhaps as an adult, he will have learned this lesson.
You get the idea. Do some brainstorming with the other boys that survive, and then write from one of these perspectives - the one that appeals the most to your creativity! Good luck.