Answer:
Improve
There are no course prerequisites to get into any accredited U.S. art college. A lot of colleges understand that there are many high schools that do not even offer art courses, so it's okay if you don't have a big list of art classes on your HS transcript.
To better your skills though, take as many art classes at your HS as you can :) Even if say you want to be a sculpture major, go ahead and take Photography I! All of the arts are related, so by participating in a new media, you'll broaden your skills even more. If you've taken numerous art classes already or if your art teacher thinks highly of you, definitely take AP Studio Art.
Oh and don't forget your core/academic classes too. Psychology is usually a good course for artists to take, or if you want to be an architect, Pre-Calc, Calculus, etc.