I went to William Woods. It's a really good school. In Missouri though. WWU | Equestrian Science Major
My other option schools I was looking at were Findlay in Ohio and West Texas A & M.
There really aren't classes in most high schools that are horse related. I would recommend anatomy and leadership type extracurriculars. Also business classes if they're offered.
Take lessons and go to shows and get videos! Findlay required a video of you with your application. Also volunteer at riding schools or with trainers for a head start.
And get good SAT scores! WWU is pretty expensive, but they give good scholarships. I was only able to afford it because I had good scores Equestrian Colleges Directory - O Horse!