Bethany and Amanda are the epitome of Mean Girls.
They're book-smart, sure, but they feel like they're more worthy of dating Jason than Macy is. Even before Macy starts her job with them at the library, they're already laughing at the questions she asks while Jason trains her to replace him. And when she reports for duty, they treat her like dirt, excluding her from their conversations, their lunches, and even from the work she's supposed to be doing: "In their minds, I didn't belong there" (3.22).
As the summer drags on, Bethany and Amanda make their opinions even clearer by acting downright hostile. And in a weird way, this behavior helps Macy move on from her obsession with perfection:
I didn't care that they snickered […], or spoke in hushed tones. Because now, I didn't care what they thought. It wasn't new, this realization that I would never be like them. What was different now was that I was glad. (10.155)
When Macy finally quits, she leaves them—and their "perfect" world—behind for good.