The rock motif is quite significant to the novel as a whole. So your assignment is a good one. You probably have already noticed several examples of rock rolling or throwing. The first major example occurs in Chapter 1 on the first expedition that the boys take to check out the island. Jack, Ralph, and Simon are on this expedition. During this adventure, they come across a big rock and send it rolling down the mountain. The boys are united here in their joy and sense of adventure:
Not for five minutes could they drag themselves away from their triumph.
Rock rolling will be repeated two more times in the novel. As it is repeated, the boys' savagery will be more intense. You will be able to see how this seemingly innocent act becomes much more sinister as tension among the boys increases and they splinter into two groups.
The other example of rock throwing occurs at the beginning of Chapter 4, in which Roger is throwing rocks at Henry, but "threw it to miss." Golding explains that Henry is protected by the teachings of "parents and school and policemen and the law." Roger has been taught that throwing rocks directly at someone is wrong, so he will only throw to irritate, not to harm. What is particularly chilling about this scene is the fact that Roger only has to realize that on the island there are no repercussions if he should throw to hit. As you read, you should pay attention to Roger's devolvement into savagery as well as his later involvement with rock rolling.