The reader must also consider the "baggage" that the boys bring to the island. Ralph is the son of an officer in the Royal Navy, and it is a general assumption that children raised in such circumstances are more mature. Ralph definitely thinks that he is more responsible. Piggy has been taught that his intelligence is most important because he is clearly not athletic. He is coddled by his "Auntie" who is raising him because of some unknown circumstances with Piggy's parents. Jack has already been given a leadership role in the choir, and there is a possibility that this clique has given in to Jack's domineering tendencies already.
They are also students, and British school culture factors heavily into their actions on the island. The "outcasts" Piggy and Simon are pushed around, the "leaders" Ralph and Jack are given respect freely, and the young'uns are protected. They are conditioned to be reserved and serious, which is made even clearer by the interaction between the boys and the Captain at the novel's end.
When we first meet Jack, he is already a totalitarian leader. He demands his choir stay in formation until Simon passes out. Later in the novel, he rules through fear. He uses Roger as his enforcer, but Jack makes the rules and demands they be followed. Because of this one could argue that Jack has not really changed at all because of the setting. Jack was peeved when Simon fainted at the beginning of their time on the island and only allowed the choir to sit down because he realized he would face mutiny if he did not. This giving in to what they wanted helped secure his position as leader, he recognized by giving in the choir was thankful to him. He did not make this move out of a humanitarian effort. Therefore it could be argued that he never showed humanity. If you hold this belief, you could reasonably argue that Jack did not change at all because of the setting.
On the other hand, one could also argue that Jack did have humanity. One could argue he was not a totalitarian leader at the beginning of the novel, before being stranded on the island, but rather a little boy who was scared and trying to keep some semblance of normalicy in a very abnormal situation. Keeping the choir in line was his way of trying to control a world that was spinning out of control (with both the war outside the island, and the fear of being through a plane crash and left without adult supervision). He showed humanity by letting the choir sit down when they arrived with the other boys despite his opposite treatment of Simon, which was obviously a common treatment thus also a semblance of normalicy. He also showed humanity by his inability to kill the first pig. If you take this second stance, then the setting had a great impact on Jack. Being forced to live without the routine that he clung so desperately to (as with the pig dance) caused Jack to degenerate. At first, this was a slow change, but once it happened, it quickened because of the effect of the setting.