Logically what she should do is stop any new students from taking on the course and allow the older ones to finish it, and then drop it. That way it's not a waste of time. Is the school strapped for cash?
You could try a petition. Get students and parents involved, try to get her to listen, and perhaps approach your local media with the story. Authority figures do tend to get a bit more co-operative or open about issues when they see a camera pointing in their face, especially with a journalist on the other end of it.
Edit: Do you mean a GCSE in general or a specific course/topic? I should have asked.