I am not majoring in music, but I was wondering what are some national piano competitions that actually distinguish one in college admissions, ie, competitions that an AO might know; competitions that will separate one from the thousands of others who play piano.
Even though you are not majoring in music, are you going to continue to play piano, develop as a musician, and contribute on campus musically?
What does your music resume look like at this point? You may not need a national competition. I can see from your user name that you are aiming for an Ivy League School and I know that many people think you need awards on a national level, but there are other (non-competitive) ways to go about things.
Others who know more about piano competitions may suggest some, so I am just coming at your question from a different angle.
When applying, you will no doubt submit an arts supplement with recording/video, resume, and letter(s) from music teachers or directors. Many things can go into that resume, if that is what you are trying to build.
You might want to look into summer programs and festivals. Or maybe you already participate. Do you participate in music outside of school at, say, a conservatory preparatory school? Do you have an excellent teacher who can help you develop and also write a good recommendation? Have you studied theory, music history, composition as well as piano? High school is a great time to have broad exposure to music, beyond technical ability.
Finally, it is not necessary to view the admissions game as a competition in this sense. No need to vie for position. Just present who you are and what you can bring to the class. Good luck.