Almost Perfect by Brian Katcher
"You only hurt the ones you love.
Logan Witherspoon recently discovered that his girlfriend of three years cheated on him. But things start to look up when a new student breezes through the halls of his small-town high school. Sage Hendricks befriends Logan at a time when he no longer trusts or believes in people. Sage has been homeschooled for a number of years and her parents have forbidden her to date anyone, but she won’t tell Logan why. One day, Logan acts on his growing feelings for Sage. Moments later, he wishes he never had. Sage finally discloses her big secret: she’s actually a boy. Enraged, frightened, and feeling betrayed, Logan lashes out at Sage and disowns her. But once Logan comes to terms with what happened, he reaches out to Sage in an attempt to understand her situation. But Logan has no idea how rocky the road back to friendship will be."
-from Random House Kids