Rusted Crowns by Raven K Storm:
 
Raelynn Occishian was only eight when a mysterious fire killed her parents; her frantic claims about ravens, blue flames, and a horned man landed her in an asylum, branded a murderer.

After ten years in the white-walled monotony of her medication-tinged reality, her only escape is her best friend, Lisa. But when a fight sends Raya crashing towards a rusted iron door, a prisoner with an otherworldly pair of icy blue eyes—and an infamous past as the Shadow Serpent assassin—entices her to free him.

The moment she obliges, her world begins to unravel. When Lisa goes missing, Raya’s world collides with the Fae realm that her best friend had long-proclaimed real. To uncover the secrets of her past and save her, Raelynn must become the deadly weapon her father had raised her to be. Reluctantly allied with the Serpent she can’t seem to shake, Raya has a king to kill and a crown to take.

Her first step? Enrolling in the Noctis Academy for Assassins, where you either graduate an Elite, or die trying.