Not having ever been to art school or worked as a photographer/artist--(I've just been someone who professionally was once handed a stack of two hundred resumes to pick three interview candidates)--take my advice with a grain of salt, but I think the first thing you should do is ask the art school in question what they want you to show them in your portfolio. Different schools may have a different emphasis on what they want to see, and simply asking may get you a better, clearer picture of what it is you need to put together. And they're very likely to have guidelines available and ready for you. And if you're applying to multiple schools, consider doing separate versions of your portfolio for each school or program, if their requirements differ.

A portfolio, for any artist, is their resume. It's how you demonstrate what you do and who you are as an artist. It's how you show your work, your voice, and your talent. "Theming" doesn't really have much to do with it. You come up with themes for a specific work, if you have to--say a photo essay or an exhibition. But a portfolio is more about the past history of what you've done and are capable of doing, and (hopefully) shows where you can go in the future.

You do have to select and arrange with an eye towards flow. You may want to arrange it chronologically to show your progress as a photographer, or you may want to arrange it by types of shooting (still life vs. landscape vs. portraits). If you've done professional work, you may want to contrast that with your personal work. But always think about it not from YOUR point of view as the creator, but from the point of view of the person who will be going through dozens of these to pick who to accept for the school. Use your viewer's hat, not your photographer's hat, as it were. And possibly consider asking a third party--friend or teacher or mentor, to help you edit your choices--they may see something you'll miss because you're too close to the work.

And if you find you're throwing things into the portfolio just to make it thicker, stop. Quality, not quantity. And go out and shoot a whole lot more so you'll know you're only showcasing the best of your work. :)

See also:

• https://claralieu.wordpress.com/2013/03/19/ask-the-art-professor-what-should-you-include-in-an-art-portfolio-for-art-school-or-college/

• http://artschools.com/resources/portfolio/