I do think she would be ready for more in language arts, and I vivdly remember my own frustration until not being challenged by a great teacher my senior year of h.s. in AP English (and then dropping an Irish poetry class in college b/c it was too hard and I just couldn't deal) and I don't want that for dd.
Picture a school that is extremely similar to this one (imagine Race to Nowhere if you don't want to read the article)--this is the environment we are talking about and why I am sure they will say that the school is full of gifted kids and they know how to challenge them. And also why in this case I am on the fence about asking for a skip (and pretty sure that they would refuse anyway):
http://www.paige-williams.com/stories/AnxietyHigh.pdf