Manga: Spell Checkers
 

I just leafed through this earlier in the book store, but the premise is that three high school witches use magic to get ahead. They're not nice, though, and (spoilers) a bullied girl resorts to black magic to get revenge.
The girl in question was a champion speller when she was younger, but was cursed during a spelling bee and lost her vocabulary skills. She turns the witches against one another, and near the end of the first volume one of them is subjected to voodoo and loses her mouth for a while. Eventually they team up again and win the day by tricking the angry girl's patron demon into turning on her, reducing her to a "shade" which resembles a life size paper doll. A scene in the second volume shows the witches toying with their new slave by making a snowman around her.