I think I read this about 5 years ago as an ebook from the library. Unfortunately, there's a lot of books in my history and I haven't noticed it on my list yet. The protagonist is a girl in high school. While in the school bathroom during a dance (I forget if there was a particular reason she was in there), she witnesses one teacher being attacked by another. The attacked teacher, with his dying breath, transfers his demon-slaying abilities over to her. As I recall it, it's a fairly typical package with her gaining the ability to detect demons and, when in their presence, gaining increased strength, speed, and durability. One of the non-combat conflicts going on is that, at the age of 15, she has her debutante coming-out coming up, and she has to spend time preparing for that rather than finding and killing the demons. I remember that there was a male love interest, another kid her age, who had his own mystic empowerment, although as I recall it one that was counter to hers. The only other thing I recall is that she had an older female teaching her to use her demon-slaying abilities, who I think she wasn't fond of from the start for some reason (part of me wants to say that they were an estranged relative who didn't really want to be in the position of teaching her).
On further exploration of my vague memories, some other things that popped into my head:
- The girl's parents are alive and well, and she gets asking with them, although one source of friction is that her demon-slaying (which she keeps secret from them) keeps making her late for her classes to prep for her Coming Out party.
- At some point in the book, it becomes plot significant that she knows that a neighbor has an empty pool in their backyard. I think she tricks a pursuer into falling into it.
- She figures out a way to tap into her powers arbitrarily, which enables her to descend the party stairway in heels
- The boy she's attracted to has some sort of prophecy powers. I think he draws them.
- Now that I'm thinking more on it, her enemy might be a set of magic-users with the demons just being one aspect.