I'm confused, why do you need a portfolio if you haven't completed any classes. Usually your portfolio is a collections of works you've done in class or jobs to show what you're capable of. I never needed a portfolio to get into a design class. But if you want to know what types of exercises. Usually in basic design classes you work on corporate identity, business logos, letterheads, business cards, etc. And typography skills; laying out brochures and other such pamphlets that show your ability to flow large amounts of text into an easy to follow and read format.

Many reputable private schools ask for portfolios, but it's hard for you to display somethng when you have never taken any classes that reflect graphic design.

I don't see how they will except you into the school without taking any graphic design classes.

You should seriously speak to the counselers there. There is a real good chance they aren't even going to look at your portfolio, and instead have you take some classes for credit to help you build that portfolio.

Nevertheless...show that you can design a Logo. Then take that Logo and apply it to business cards, letterheads, envelope. You should know how to use illustrator and photoshop to do this. I doupt hand drawings will be acceptable.

You should also create an imaginary page layout (like designing the inside of a magazine page), that has images and text..this will show them your typography and compositional strength.

Along with that, show your drawing or painting talents, whatever you have..that shows you are artistic and creative. Around a dozen total pieces would be great!!

I still strongly suggest talking to a counselors and having them show you some samples of what they are looking for. Then start working on your portfolio. Don't waste your time...know what that schools looking for, and make your portfolio accordingly.

Good Luck!