This whole sordid tale of The Disreputable History of Frankie Landau-Banks is told from the perspective of an uninvolved third-person narrator who happens to know everything that's going on in one character's head: Frankie's.
The narrator tells us everything we need to know about the layout of the school, the way the social hierarchy is set up, and more. But in addition to the mere facts of the situation, the narrator tells us how Frankie is feeling and thinking. We can literally read her mind as she begins formulating her schemes and plans. We get to see how she feels about everything. So we know that while on the outside she looks perfectly happy, inside she's actually fuming about the fact that she knows her boyfriend is lying to her. Without all these details, we would never get to know Frankie as well as we do.
For example, even when Frankie is seemingly nonchalant about Star's breakup, we get to see how she's taking it on the inside:
Frankie nodded, but she wasn't thinking about Star.
She was thinking how easy it would be for the same thing to happen to her. (23.20-21)
Even though Frankie seems composed and naïve on the outside, her mind's always working on overdrive. And we get to see some of those cogs in motion.
So if the narrator is sticking so close to Frankie, you have to wonder why Frankie isn't just telling us this story in her own words. In Shmoop's humble opinion, that's because having a third person narrator allows the book to keep a healthy distance from Frankie when it needs to. We know before she does when Frankie's just fooling herself about her relationship with Matthew, and we know there's more than a little spark between her and Alpha, too (who's hoping for a sequel?).