If you are interested in applying to a special school, like a charter school, a college, or a graduate school in the next few years, you should keep a full record of your academic achievements.
A portfolio is simply a collection of documents, materials, and records. An academic portfolio would be a record of your achievements in school, and the documents would include things like school tests, research papers, science fair abstracts, newspaper clippings, awards, and more. You can keep this collection in a binder, folder, or briefcase, and you can make it as fancy or as simple as you like.
The purpose of a portfolio is to keep records together to tell a story about your achievements. As you apply to special schools, you might find that some actually require a portfolio, while other schools just want a description of your achievements. The earlier you begin to assemble your portfolio, the more information you'll be able to provide. This will make you look very good on an application!