Half-Blood. Book 1. Jennifer L. Armentrout

Three years have passed since seventeen-year-old Alex's mother took her from the Covenant, a school where pure-blooded descendants of the gods hone their abilities and half-bloods like Alex learn to slay daemons. After the creatures track down a runaway and kill the girl's mother, she returns to the Covenant, where she is faced with a choice: become a servant of the pure-bloods, or train hard to catch up and continue her studies.

The second option seems obvious, but everything becomes complicated when Aiden, an extremely attractive pure-blood, takes on the task of personally training Alex. And that's not her only problem. The girl learns the real reason why her mother hid her, and when daemons begin to hunt under the walls of the Covenant, Alex uses all the skills she has acquired to survive.