For the topic civilized vs. savagery, you could write your essay about how savagery overcomes civilization because the boys are so young. You can use the following topic sentences and quotes:
1. Though the boys try to establish some sort of system, it does not take a long time for the boys to quit listening to their chief.
Quotes #1: "We've got to have rules and obey them. After all, we're not savages. We're English, and the English are best at everything." (2.192)
Quote #2: "All this I meant to say. Now I've said it. You voted me for chief. Now you do what I say."
They quieted, slowly, and at last were seated again. Ralph dropped down and spoke in his ordinary voice." (5,58-59)
2. When Jack goes to hunt, he immediately gets overexcited from the thrill of injuring and killing the pig.
Quote #3:"He [Jack] tried to convey the compulsion to track down and kill that was swallowing him up.
"I went on. I thought, by myself—"
The madness came into his eyes again.
"I thought I might kill." (3,37-40)
Quote #4: [Jack] began to dance and his laughter became a bloodthirsty snarling. (4,33)
3. Simon's and Piggy's deaths symbolize how savagery's wins over civilization:
Quote #5: "The sticks fell and the mouth of the new circle crunched and screamed. The beast [Simon] was on its knees in the center, its arms folded over its face. It was crying out against the abominable noise something about a body on the hill." (9, 89)
Quote #6: "The rock struck Piggy a glancing blow from chin to knee; the conch exploded into a thousand white fragments and ceased to exist." (11, 209)
I do not know which edition you are using, so the quotes are identified by (chapter, paragraph).