I remember seeing a preview for this movie, it was set at an all girls school. At the school there was one teacher that was admired by students the most. She was the most glamorous and she was also the swim coach. When a new (maybe french?) student comes to school she becomes the teacher new favorite student and all other students are jealous. Some how the teacher convinces the other students that they should kill the new girl. I think it's set in the past, the title may have something to do with "glass". hope these hints are enough.