@JaneLittleH: So, if this daughter is in 7th grade, she is not the one admitted to Exeter which you are considering turning down because you are unimpressed by the headmaster and because you “want the kid to be close to home” as posted on your other thread. Why are you considering boarding school for this younger child then?
All of the boarding schools discussed in this forum will give your daughter a great all-round high school education, including extensive experience in the fundamentals of good writing, but they are not in the business of turning out novelists or any other specialty; there are no “majors” in boarding schools. Some BS are stronger in certain areas than others, and some are solely arts focused as noted above but, if you are questioning the value of Exeter, I’m not sure any suggestions for BS with good writing programs will satisfy you.
I completely agree with @Dustyfeathers #7 and would re-read that post several times and evaluate high school and college programs based on the strength of the broader education they provide. There is plenty of time for this young child to develop into a writer if that is what she is meant to be or to follow other pursuits as she discovers them. If she is a writer, she will write; there is no escaping that calling.
Anecdote: Our son was hell-bent on film school for college and chose his boarding school based on the ability to expand this interest and pursue a film career. Before entering BS at 14, he had twice taken first place in a film category at a major university’s annual digital film competition. He ended up graduating from a service academy with a degree in electrical engineering. Kids change a lot during their high school years. Now is not the time to pigeon-hole them.