First, this sounds like a most interesting and challenging assignment. (Congratulate your teacher for his/her creativity.) Secondly, I think your suggestion about Roger murdering the marked littlun is an excellent idea and may be a better choice than anything I can suggest. Just thinking off the top of my head:
- What if an adult had survived (the pilot, for example); he would be the chief. Would the boys have followed him or revolted in much the same way they did?
- What if Ralph had been a bit friendlier and more receptive to Piggy's personality and ideas? What would have been his fate?
- How about a continuation of the ending? What kind of prologue would you write?