Mortal Coils Series by Eric S. Nylund

"Nothing interesting had ever happened" to orphans Eliot and Fiona for 15 years with strict Grandmother and well-meaning great gran Cee. But unintended offspring of a goddess and Lucifer can break a long truce. Rather than kill the pair, Immortals create three Trials; Infernals add three Temptations. The pair must stick together; adult allegiances are uncertain.

Eliot and Fiona Post are twins whose parental clans are foes. Mom is Immortal goddess Atropos, eldest Fate. Dad is Infernal Lucifer, Satan. High School is Paxington, where gods and goddesses, warriors and sorcerers, learn to harness their powers. Classmates are allies in battle or threats to your life. To flunk is to die. Only the toughest graduate.