Is it? A lot of people seem to have come to the conclusion that its characters are realistic.

The novel is styled as allegorical fiction, embodying the concepts of inherent human savagery, mob mentality, and totalitarian leadership. However, Golding deviates from typical allegory in that both the protagonists and the antagonists are fully developed, realistic characters.

https://en.wikipedia.org/wiki/Lord_of_the_Flies

Golding’s work is a powerful exploration of the inherent capacity for savagery within human beings when societal structures are removed. The novel touches on themes such as the loss of innocence, the struggle between civilization and savagery, and the fragility of societal norms.

https://www.sparknotes.com/lit/flies/

it serves as an interesting look at the dark side of human nature and how no one is beyond its reach. Plus, anyone who had a bit of a rough time in high school will probably not find the events in this book a huge leap of the imagination

The scary thing about this book is how real it is. The Lord of the Flies bespeaks the brilliance of realistic dystopian fiction, it gives you a possible world scenario, a bunch of very human characters and then it shows you want might happen when they are thrown into a terrible situation: they act like monsters (or humans?)

the author very realistically portrays human behavior in an environment where civilization no longer has meaning.

If you have never experienced the amount of destructive power that is possible in that short time-span, you might think Golding exaggerates. Unfortunately, I can see any group of students turning into the characters in The Lord Of The Flies if they are put in the situation.

https://www.goodreads.com/book/show/7624.Lord_of_the_Flies

If it's not supposed to be realistic, he did a shitty job of communicating that.