Why does Susan agree to Marks lie? Why does Mark want her to lie?

I know this is long, but bear with me. This might even be good for a quiz Open-Ended:

Mark is very manipulative, as the story shows. He studies the reactions of people and how they act in certain situations. Mark realized that Susan had a small thing against Mr. Griffin, saying that she's used to getting A's insead of B's, making it easier for Susan to get hooked in the plan. However, it wasnt just because of Mark. She also chose to be in because of David Ruggles; Susan's weak point. Mark wants her to lie beacause he doesn't want her to reveal the favt that Mr. Griffin is dead on their behalf.