The hidden knife / Melissa Marr.

Enhanced descriptions from Syndetics:

"Unique and gripping."--Tamora Pierce

New York Times bestselling author Melissa Marr invites readers into a magical world where stone gargoyles live among humans, ferocious water horses infiltrate the sea, and school hallways are riddled with magic wards--and where a group of young heroes seeking justice discovers those very creatures are the best of allies.

Twenty years ago, a door opened between the world of humans and the Netherwhere, allowing all kinds of otherworldly creatures entry. Some, like the kelpies and fairies, who like to bite, are best avoided. But the gargoyles are wise and wonderful, and show a special affection for humans.

Vicky has grown up under the watchful eye of a gargoyle named Rupert, and excels at sword-fighting and magic. But there's so much she doesn't know--like why her mother, once one of the queen's elite Ravens, keeps Vicky hidden away and won't let Vicky train at the elite Corvus school where girls with her gifts perfect their skills. But when a horrific tragedy occurs, Vicky knows it's finally time to use her gifts, and that the only place she should be to avenge the crime against her family is at Corvus. There she bands together with a former street thief and an alchemy student to figure out whom they can trust in a place that's rife with intrigue and secrets. And all the while, the gargoyles watch and nudge. Time's not linear to them, so they know change comes in ripples. With their steadying influence, Vicky and her friends just might be the generation to expose the court's secrets and ensure a better future for both worlds.