"The top" surely refers to the top of the mountain that Jack, Ralph, and Simon climb.  To me, there are a couple of things that might be what your teacher is asking for.

1. They think that what they are doing is really cool.  They call it "wacco" and "wizard."  They think the exploration is fun and they start wrestling with each other because that's what boys do...
2. They push that huge rock off the cliff and watch and listen as it falls.  That sort of foreshadows something important from later in the book...

Look at your questions and see if you can tell which of these answers is more likely to be right.