What big part did Jack play in Simon's and Piggy's death?

Hey there. I think you need to argue that Jack was the reason things escalated to the point they did. He lured all the boys to his group and his violent ways were infectious. He handled disagreements through violence and intimidation. Consider how both boys died and you will get the picture.

The problem is, I didn't read the chapters, so I don't know what happened to Piggy and/or Simon.

I'm pasting a link below that will take you directly to a complete text of the book. You might also want to take a close look at gradesaver's study guide for the novel. You do have to do some reading in order to complete homework assignments, especially when writing an essay. Good Luck!

The assignment you are doing pretty much demands that you read the book. If there is a real problem I suggest you at least go to the GradeSaver summary.

In what chapters did this happen?

Simon dies in chapter 9 and Piggy dies in, I think, chapter 11. Below is the summary link,