Relatively few high schools have design programs, so a lot of incoming portfolios will be similar to yours. But if you want to add some design, then consider picking a course or two from Udemy, Udacity, edX, Skillshare, etc. You'll learn some great foundational skills and you'll end up with some design work for your portfolio. Everybody applying will talk about how important design is to them and how much they want to learn about it, but you'll be proving this by your initiative. Action expresses priority.
Here are a couple free ones that I think look good:
Also, start thinking about your essay! Showing that you want to study design is essential for the reasons I just said, but don't mistake that for meaning that how you speak and explain yourself doesn't matter. It does. It gives context and allows you to make a personal connection with the reviewers. Why do you want to study design? Why does design matter to you? Why is the CMU School of Design your dream school?
Since you've got a bit of time, I think it'd be smart for you to pick up a couple books about design. Here are some good options:
Pick whichever two or three seem most interesting to you. Skim through them quickly without letting yourself get bogged down (but maybe take a few notes for anything new and profound that you learn), then go back and read them slowly/properly if you have time. And remember, used copies are just as good as new but cost less, and libraries cost nothing at all.
Good luck!
Check out The Inmates Are Running The Asylum it is full of examples of poor design: