OK. To begin, the concept is awesome and deliciously chilling. If you don't mind, I'll take a rating approach to things, and address the issues as I go along.
Spelling and Grammar: 7/10
Your spelling was pretty good. I noticed very few errors. Your grammar, however, could use a little more work. If you have specific grammar questions about a particular section of your story, I'd be happy to help with that.
Character Personality: 3/10
I found your characters to be rather flat and mechanical, though you did make an attempt at making them believable. I would suggest that you don't explain every thought and detail about the character to the reader; let it be gradual.
Imagery: 7/10
You did rather well at imagery. Especially in the scene where Liby's leg is being removed; I really could see that. How you added the sight and feel silk of her dress and the feel of the chainsaw gave a good picture.
But at times descriptions were too much. (E.g. Darker's first day at school: when the student's poured into the classroom, I felt like I was being force fed their descriptions, and at times, their personalities. Sure, tell me the color of a character's hair, but I don't need to know what shoes he's wearing; don't give the reader too much.)
At other times descriptions were too vague. For one, I had no idea what age range Darker was teaching, at that unknown fact grew even more obscure when Jane was described in the newspaper as a 'little girl,' and then later as a young woman.
Character Speech: 5/10
I mentioned this before, the character's are too mechanical. The woman at the school that Darker saw on his first day, for one. She mentioned that she was shy. I found that piece of information awkward, but not in a way that added to the story or her character. It didn't belong.
Your characters often said either too much or too little throughout the story.
Gore: 9/10
Almost a perfect score for gore! Bloody, dude, bloody. The mention of cartilage made my toes curl; nice.
Horror/Suspense: 2/10
Yup. It was almost comical how Liby and Jane reacted to Clownface. Sure, they bad-talked him, but complied willingly. I'd cap it up to fear, but I didn't sense any from either girls. Also, the action happened too quickly. Boom, a leg. Boom, Clownface left the room. Boom, Clownface is back. Give the reader time to breathe every now and then.
Also, whenever a girl lost a body part, she seemed okay afterwards, minus the blood. It was as if they weren't in any pain. I mean, dude, they would've blacked out eventually, possibly during the receiving of their first wounds, definitely during the removal of their limbs.
I did like the idea of the game, though; pure evil.
Final Score: 33/60
I would be absolutely happy to give you tips on editing the story. I could even edit small sections of it if you needed the help. Grammar and spelling-wise and all that. I'm wary of giving story/plot advice except in regards to structure, as I want the storyline to remain yours.
Clownface: This guy gives me the creeps; he's absolutely perfect. This question is unrelated to the storyline, but what is his motivation for doing this? Is he just crazy? Did something personal happen in his life to make him act this way? Did he think too deeply on the problems of the world and decide he could make things better in some sick way? I'm just curious.
My thoughts over all:
Love the concept. Wince at the delivery. Hope I haven't been to harsh.
~Peter