This is very Sara Shepard, which is practically its own genre now, but I enjoyed it about as much as I typically enjoy her books. I don't want to say that she's a guilty pleasure, because that suggests that I ought to feel guilty for anything I read (I pretty much don't at this point), but this is the kind of thing I suck down like a crazy person without bothering to come up for air until then end, but then rarely bother recommending to people, really. It is probably my equivalent to tv for most people.
There are your standard twists and turns and sudden, shocking moments. Unlike The Heiresses, which I read and reviewed recently, this is very much a YA book. The high school setting works well for it, and also gives the main characters a reason to not necessarily want to trust going to the adults every time something creepy happens; teenagers just don't do that, even when they should. Even when they are smarter than these girls are, and to be honest, the intelligence for several of these characters was more of an informed trait than anything actually displayed at any given point.
They have an even more valid reason not to go to the police because they are aware they really do look suspect. This isn't the type of thing that would be easily played off or explained, so it's a completely valid concern beyond teenage hi-jinks to remain quiet about what they do know and try to solve it themselves before they end up in a whole lot of trouble.
This feels very much like Pretty Little Liars in a lot of ways, and given how well I feel Sara Shepard does that type of thing, that's not really a problem. It is a quick, compulsive read, and clearly the beginning of a series, given things are not completely wrapped up at the end. There are more obvious suspects than seems possible, and there are probably a few more waiting in the wings for the sequel with motives we can't even guess at at this point. I'll probably read the series in its entirety, and if you're looking for something fluffy and page-turning, it's not a bad read, though I would start with Pretty Little Liars if only because it is complete.