Occasionally prep schools will admit a non-athlete for a PG (post graduate) year. Rather than repeat a year & rather than disrupting the sequence of your current high school curriculum, finish high school & reassess as to whether you would like to progress on to college or to apply as a PG for a post graduate year of further high school studies.
Based on the limited information shared in your original post in this thread, I think that you should graduate from your current school and consider a gap year to study abroad as a PG or to teach English in a non-English speaking country.
The difficulty in suggesting options for you is that it is not clear as to why you want to attend Exeter as it is clear as evidenced by your GPA that you are currently being challenged academically in your Catholic school.