Hi, Megan!
First of all, congratulations on your upcoming high school graduation! As a high school teacher with four classes of seniors this year, I know first-hand how stressful your last year and a half must have been, so a trip to Walt Disney World is the perfect way to celebrate!
Aside from
celebration recognition buttons, which you can obtain at Guest Services in the parks, you might want to look for pins which have "2013" on them. They are very sturdy and are a great way to mark the year of your trip (and, in your case, to help celebrate that you are a proud member of the Class of 2013.)
Moreover, there are graduation Mickey Mouse ears, as well, usually with the year of your graduation embroidered onto it, too. When I head to WDW with several hundred seniors (yes, that is not a typo), these graduation ears are among the top purchases of my students. You'll love them!
If you are afraid that there may be none left by the time to get to WDW in August, call Disney Parks Merchandise Guest Services now at 1-877-560-6477. A Cast Member there can let you know when those ears will be available in the parks and can even assist you with purchasing them over the phone and having them shipped to your home. That's the best way to ensure that you'll have these very popular items while on your celebratory trip.
Again, congrats, Megan! Have a fantastic end of your senior year; be sure to send more questions as you think of them, too. I love to help!