Don't have much to go on as it's a good ten years since I last read it, but...
It started with a young girl who met some other children at a Christmas party, and they put all of their birthdays in a book? And then talked the other children into bullying each other on their birthdays?
That's all I remember, and google is failing me.
If anyone has a clue what I'm on about, I'd be impressed grateful! 