I have to admit, when I was a kid (and I was a pretty rule-abiding and easy kid), I imagined a world much like the one in Lord of the Flies. No adults telling me what to do. No homework. The ability to be outside all day and do whatever I wanted. This seemed pretty perfect to me.
Of course, to obtain that ideal world, I would have to make some pretty significant sacrifices. And if I thought about it further, I couldn't simply play all day in a world without adults and have food to eat when I was hungry. I couldn't enjoy things like electricity because adults were the ones who took care of all those little luxuries.
This, then, is the impossibility that any imagined, perfect (Utopian) society faces. In a perfect world, there is only goodness. People are only happy. There is never any conflict. At first thought, all of these things sound, well, pretty perfect.
But if people are always happy, how do they react when something bad happens? How do they react to death? With happiness?
A Utopian society is impossible because in order to create a "perfect" world, citizens have to give up a lot of personal freedoms. In some pieces of literature, the government has taken complete control, even over emotions. In other books, citizens have memories that don't allow them to recall things that aren't pleasant. And in others, the society is so strictly organized that people are only allowed to experience superficial happiness.
How does all of this apply to Lord of the Flies? This all means that a Utopia isn't possible, so by default, all societies that strive for this perfect state are instead dystopian. On the island, it may seem like a perfect society for kids to live in a world with no adults and no rules, but their society quickly degenerates into savagery and murder (certainly not ideal in a "perfect" world). The boys split into factions that cannot get along, and there is a growing conflict between them throughout the novel with Jack looking to force his way into a position of power, even if he has to kill everyone in his path.
Lord of the Flies is a dystopian world because a perfect world simply isn't possible.