Uncanny Valley Girl

Good heavens! Is that blood on that knife? And why is she smiling?

She's cute. Beautiful, in fact. Friendly and popular. She's the head of the class in terms of looks, money and everything that counts for her approval rating, but she's not the Alpha Bitch, even if she has a Girl Posse or the entire grade on her side. One of Those Two Guys might have a crush on her, even though he never gets anywhere. She's the perfect girl...

...except there's something not quite human about her, and she's secretly out for your blood.

The Uncanny Valley Girl is smarter than she lets on, and puts on a very convincing persona that's made the class adore her. It's the high school girl version of Villain with Good Publicity, except the publicity is good enough that nobody, not even The Hero, knows she's the villain—or anything other than normal. Not until she comes at you with a sharp object, that is.

Compare Yandere, Cute and Psycho, Stepford Smiler, Living with the Villain, Dark Magical Girl. Contrast Alpha Bitch, Fallen Princess, Loners Are Freaks.

Note that the name comes from Uncanny Valley, not Valley Girl.

Examples of Uncanny Valley Girl include:

Anime and Manga

Ryoko Asakura of Suzumiya Haruhi -- the Uncanny Valley Girl. She keeps up her charming Yamato Nadeshiko persona and amazing smile even while trying to gut Kyon.

"You think it's a... joke? Huh. You don't wanna die? You don't want me to kill you?"

- As of the tenth novel, Kimidori Emiri has revealed herself to be every bit as apathetic towards human suffering as Ryoko.
Mio Hio from D.N.Angel fits this to, like, a really creepy T! She's an American Expy, even has one of Those Two Guys when she's after the other to either kill him or bind his Alter Ego Phantom Thief. Death Equals Redemption, which is, like, a drag.
The Vampire Princess Miyu OAV give us a schoolgirl named Ranka. She's pale, beautiful, elegant, apparently a perfect Yamato Nadeshiko who has managed to ensnare the heart of the local Chick Magnet... and she's also a Shinma that transforms people into dolls. And her guy knows it and loves her that way.
- To a certain degree, Miyu herself counts when she poses as a schoolgirl to fit better in the human crowd.
- Did we forget how Chisato Inoue became one of these after her Shinma awakening and her Face Heel Turn? Brrrrr...
From My-HiME, there's Miyu Greer's adorable little charge/soulmate, Alyssa Searrs, the Golden Angel of the School Choir.
Johan Liebert from Monster decides to pull a stint as an Uncanny Valley Boy in volumes 6 through 9, when he starts attending University of Munich as part of a plan to get at Hans Georg Schuwald and puts up a front of the most mind-bogglingly perfect human being in existence. When Schuwald is finally confronted with the possibility that his new student secretary is a psychotic bastard that's probably planning to kill him, Schuwald isn't even the least bit surprised. Why? Because he was so damn perfect that he just had to be evil.
The mysterious transfer student in Saitama Chainsaw Shoujo seems pretty unfazed when she encounters the blood-drenched, murderous protagonist... complete with giggling and smirking while throwing her "boyfriend" towards Fumio's chainsaw.
Haruna Niekawa from Durarara!! Though it's mainly due to her being possessed by a Yandere Living Weapon.
Ryuuko Kounuma from Wolfen Crest. She seems nice enough, is rather charming and carefree... Except she is completely psychotic underneath. Having spent several years under the sexual abuse coming from her dad and her pimp, she deals with it by having sex with lots of men and ruin their lives ruined afterwards, to an insane extent. She dosen't fit the Emotionless description, though, as she's pretty much Ax Crazy.
- In an unusual twist, she isn't the scariest character. It's her boyfriend, Haguro Dou, who fits the relatively uncanny part of the trope quite perfectly. That's a whole different can of worms, though...

Comic Books

In X-Men, the seductive Foxx turns out to be the shapeshifter Mystique trying to seduce Gambit away from Rogue. A similar plot occurred in X-Men Evolution, when Rogue's friend Risty Wilde turns out to be Mystique spying on/stalking her adoptive daughter.

Film

In Basic Instinct, Sharon Stone's character Catherine Tramell is this. She is a beautiful, wealthy rock star but embodies this trope with her fondness for ice picks
Alice from Transformers: Revenge of the Fallen. She looks like a hot blonde who can't keep her hands off Sam, but she's actually a Decepticon who's after the Allspark info in Sam's brain.
Marybeth from The Faculty.
Gwen in Sky High.
Jennifer's Body. 'Nuff said.^[context?]

Literature

Tammy manages to conceal the fact that she's the high priestess of a cult trying to bring about the end of the world, and the one responsible for the plague of zombies in "Gil's All Fright Diner" , despite that the townsfolk (as well as the drifter two main characters) are VERY experienced with the supernatural. This may be because she uses her great female beauty to distract and manipulate all males in the book. The fact that she only has one cultist to aid her (her would-be boyfriend) and is a teenager probably doesn't hurt, either.
In Percy Jackson and The Olympians book 4, Percy starts off the book by meeting Kelli and Tammi, two beautiful cheerleaders. They later turn out to be monsters concealed by the Mist, and they try to kill Percy. Oh, and they're also part of Kronos' army.
In the Uglies series, the "Specials" are essentially this: Tally describes them as being very beautiful, but a mean, threatening kind of pretty.

Live-Action TV

Subverted on Ned's Declassified School Survival Guide with May, June, and Julie, the "Stepford Schoolgirls". Moze becomes convinced that the three girls are robots, but they're not; there's a perfectly logical (read: badly executed) explanation for everything.
Kuroki Mio in Pretty Guardian Sailor Moon.
Debbie Berwick (self-described Cupcake Queen of the Universe) in Phil of the Future is an evil robot. But until the "Halloween" episode, only Pim seems to see it.
Ingrid from Young Dracula, though it takes a major Weirdness Censor on the part of everyone else not to notice.
Darla from Buffy makes her introduction as one of these in the first episode.

Video Games

An old text game exists where the Big Bad (though you didn't know she was a Big Bad at that point) was a woman whose figure was described, to begin with, as "impossible" or "implausible". And you could sleep with her if you wanted, but then she turned out to be a monster that fed on Life Energy and you get a Nonstandard Game Over.
Tsukihime, a stand-out Visual Novel, can feature more than one of these depending on the route you take. The stand-out example, however, is Kohaku. On first appearence she's the one ray of cheerful Meido-shaped sunshine in an otherwise cold and unwelcoming Big Fancy House; but delve into the Far Side routes, and when you see her real smile... eep.

Web Comics

You wouldn't believe it from how she acts around the other protagonists (so it's not exactly a straight example), but from the perspective of her schoolmates, Mell Kelly of Narbonic would probably count as one of these, since when she's at college she puts up a facade of quiet sweetness, as seen in the "Mell Expelled" storyline.

Western Animation

In the episode of W.I.T.C.H. "T is for Trauma", Big Bad Nerissa disguises herself as the beautiful Stacie, showing up at Sheffield Institute in order to lure the heroines' boyfriends away and to spy on them. In addition to being beautiful, she uses a spell to make every guy in school completely crazy for her (except Will's boyfriend Matt, who was immune because Nerissa had already messed with him).
Invader Zim could be seen as a brutal subversion of that trope, except that he's not a girl. Tak (who is a girl) is a somewhat straighter example.
The Joo Dees in Avatar: The Last Airbender.
In The Oblongs, Milo gets a crush on an alien who infiltrates not only his school but also the Debbies' clique, despite her Paper-Thin Disguise.
As mentioned, Risty aka Mystique in X-Men: Evolution.