What's the Name of That Book??? discussion

This topic is about
In the Serpent's Coils
SOLVED: Children's/YA
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SOLVED. YA fantasy/supernatural, boarding school, witches. Potential Spoilers ahead. [s]
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Here goes nothing.
So I read this unfinished series when I was in high school (somewhere between 2009-2012). The one thing that I can remember very clearly about it is the cover, which almost always featured a girl or a symbolic image, surrounded by some kind of fog/smoke, almost as if you're looking at the image through a crystal ball, which I suppose was the point. It didn't look that old, and I think that the author was female, though truth be told I'm not sure on that one. I do know that I read this in paperback form, however.
Some basic plot points that I remember are that this girl is sent to a finishing school/boarding school by her older brother, the boarding school is run by witches, and that this girl gets involved with a group of friends, and that their code to meet after curfew is asking the leader if they can borrow a stocking. I know it's not much to go off of, but this has been driving me nuts, so if anybody would happen to know what it is, please help me regain my sanity!
Additional information: I do know that the series was incomplete, at least in about 2010/2011; I'm pretty sure it was something to do with the publisher, and not because the author had died or anything.