Mortal Coils, book 2: All That Lives Must Die
Eric Nylund2010

Eliot and Fiona Post are twins caught up in an epic custody battle between their mother's and father's families. Their mother is the immortal goddess Atropos, the eldest Fate, and their father is Lucifer, Lord of the infernals, a diabolical fallen angel. The families have put them through rigorous, life-threatening challenges, and together they've risen to combat them in amazing ways. But now they are facing the greatest trial of all - high school.

Paxington Unviersity is no normal high school - it's a place where gods and goddesses, warriors and socerers learn to harness their power, where a debate in class can end in a duel, and your classmates aren't simply friends or enemies, but allies in battle or threats to your life. To flunk is to die--only the toughest graduate. As Fiona and Eliot struggle to keep up their grades by surviving the rigorous training, both families are watching.

High school is bad enough, but imagine being caught in the midst of an immortal/infernal war...