The Duff by Kody Keplinger

Seventeen-year-old Bianca Piper may not be the prettiest girl in her high school, but she has a loyal group of friends, a biting wit, and a spot-on BS detector. She's also way too smart to fall for the charms of man-slut and slimy school hottie Wesley Rush, who calls Bianca the Duff—the Designated Ugly Fat Friend—of her crew.

But things aren't so great at home and Bianca, desperate for a distraction, ends up kissing Wesley. Worse, she likes it. Eager for escape, Bianca throws herself into a closeted enemies-with-benefits relationship with him.

Until it all goes horribly awry. It turns out Wesley isn't such a bad listener, and his life is pretty screwed up, too. Suddenly Bianca realizes with absolute horror that she's falling for the guy she thought she hated more than anyone.