You probably thought of this when you picked up this book, but Skin Hunger is a sorta strange title. Why would a book about resurrecting magic be called that? What do skin and hunger have to do with magic, a fantasy setting, or a young adult novel? Well for starters, all of the characters experience hunger at some point in the book, whether it's caused by poverty or torture.
But there's a more direct reason behind the title. We finally figure it out near the end of the book, when Hahp has this exchange with Gerrard:
He took another step toward me. Then he stopped. "If I help you," he whispered, using barely enough breath to shape the words, "will you help me?"
I nodded, astounded. "Yes."
"And then will you help me destroy this place?"
I knew he couldn't possibly keep a promise like that, but I put out my hand, and he gripped it. The touch of flesh on my flesh, his skin on my skin, jolted me into feeling a kind of hunger I hadn't even recognized. How long had it been since I had touched anyone? (64.21-24)
Hahp realizes that the wizards have been deliberately keeping the boys isolated at the academy, keeping them from forming friendships or alliances. In addition to the emotional isolation, there's a physical separation between the boys, and they're never encouraged to touch (like by playing games or shaking hands). Lacking a sense of connection with one another, they also lack empathy for each other, which enables the wizards to make the boys complicit in starving and competing against one another.
The whole isolation theme applies in Sadima's storyline too, though less literally than in Hahp's. Sadima works to end her loneliness by seeking out Franklin since he also has abilities that no one else possesses. But her feelings for him allow her to be trapped in the situation by Somiss's machinations. Even though she and Franklin are physically close to one another, they don't get to be affectionate with each other very much, for fear of Somiss catching them. So Sadima, too, is hungry for skin-on-skin contact and the emotional closeness that comes with it.