I think one of David's friends goes and talks to her and mentions something about David, maybe saying that David likes her or will go out with her if she helps.

Even if they didn't tell Sue that David liked her, Sue would do it, because it would help her to fit in and she would hang out with all of them.

I noticed in Chapter four that Mark, Jeff, Betsy, and David all make Susan feel like she has been a part of them forever. They have never talked to her at school, and probably never would've talked to her period until they need her for something. Since she is least expected to rebel against a teacher, they use her as a decoy.

She's the bait. They use her apparent lack of self confidence and her puppy love of David to get her to do what they want...sickos.