I don't teach at any of those schools but I do review incoming portfolios and maybe this will help.
Most schools emphasize drawing from direct observation as their primary basis for the portfolio, no matter what aspect of art you want to study. That's because basic drawing skills are fundamental and because drawing is a little more likely to be a uniform measure than other areas of art and design.
Read the school's suggestions for portfolio submission carefully. Most will say "10 to 20 pieces" and I can tell you that more is often not better. If you have ten really strong works to submit, and then the quality level noticeably drops, better to show ten uniformly good works than a whole range.
Ask your art teacher to help you because students can get too attached to a work for sentimental reasons. Don't think all the work for your portfolio must be "finished" because showing some pages of studies from your sketchbooks can often be more informative than showing a final piece you've labored over.
In seeing for yourself, are there regional portfolio days you might be able to attend? Then you can get a sense of the range of work from other students at other schools. Where do you stand in relation to the students at your own school?
One thing I'd really encourage you to do is make sure that you are focusing on your academics as well as your portfolio. I have rejected good portfolios from students who have poor or limited academic credentials, and I've accepted weaker portfolios from students with high academics and limited art classes. Filling every possible slot of your high school time with art classes is a huge turn-off if I am considering you for my program. If you have proven that you have a brain and you are willing and able to use it for diverse learning, I can teach you to be a better artist and I can bet that you will adapt and flourish in higher education better than someone who only wants to work on art.
Finesse your writing skills, master basic sciences, history, math, etc. Do well with a foreign language, take public speaking, and in general be well rounded because in the long run those things will contribute to your success in the arts.