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.
The question is asking you to take on the "voice" of one of the characters. This means that you have to observe the personality of Ralph or Jack and write a final chapter. I can't write it for you but here are two strategies to consider.
Ralph
"We are saved but really we aren't. I have lost Simon and Piggy and everything that was good on that stupid island. I tried my best, really I did but that Jack; he ruined everything..."
Jack
"I didn't want rescue. I was a king on that island; now I am nothing. I'm a kid going to counseling and I hate it. I blame Ralph......."
Good luck!
Do I have a re-write the chapter or do I make my own using what the strategies you give me?
For your question, I would not re-write anything. Your teacher wants you to write a final chapter to the book from the point of view of Jack or Ralph. So, consider what one of these boys might be doing after the rescue. Where might they be? Would Ralph be back at school? Would Jack be in some kind of Youth prison or in therapy? Then try to establish some level of summary and closure to the book using their "voice" and point of view.
What is voice and point of view?
"Voice" is the character's personality that comes out in the writing. Point of view is simply from which person's eyes you are seeing the story through.
