While it's impressive that a boarding school offers subjects beyond typical AP classes, after browsing the course information it seems that they are merely watered down versions of what is available at universities. The vector calculus course covers in one year what is usually covered in 10 weeks at some universities. The analytical chemistry class doesn't even touch on complex equilibria, instead choosing a shotgun approach of various topics.
OP, you would be re-taking these classes anyway. You would be much better served taking courses at a community college and then entering university 1-2 years earlier than expected than taking watered down classes you would be re-taking anyway. At a community college you can easily knock out your general education requirements while taking pre-med requirements such as physics, chemistry, organic chemistry, biology, anatomy&physiology, etc.