High School Course Suggestions
Art
Any design work that will need to be done will require skills in color theory, typography, and interface design, skills that are learned in a basic way through high school art courses.
Artistic talent and skill are of major importance when trying to do well in your degree level program as well as once you get to the work world.
English
No one likes a designer that misspells common words as it cheapens the overall effect, even if you are an amazing designer.
The more advanced you take your understanding of the English language, the more ability you will have to work on full advertising creatives, as well as much more detailed mock-ups that will impress your professors and employers all that much more.