The DUFF by Kody Keplinger

Seventeen-year-old Bianca Piper is cynical and loyal, and she doesn't think she's the prettiest of her friends by a long shot. She's also way too smart to fall for the charms of man and slimy school hottie Wesley Rush. In fact, Bianca hates him. And when he nicknames her "the Duff," she throws her drink in his face. But things aren't so great at home right now, and Bianca is desperate for a distraction. She 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.