Nightschool

From Yen Press:

Nightschool answers the age-old question—Where do demons get their diplomas? Schools may close for the night and the lights may be off, but class is still in session. At the Nightschool, werewolves, vampires, and witches gather to learn everything from spell casting to calculus. Alex, a young weirn (a particular kind of witch) has always been homeschooled, but dark forces seem to be drawing her closer to the Nightschool and the mysteries within.

[The series is finished for now. But the author hints at a possible sequel.]