Yeah, if you're set on the white dress, you can wear a slip under it... or, probably, a skirt slip and a matching camisole (they make strapless things too if necessary), because it looks like your top may be a few sizes larger than your bottom. (I hope that's not rude to say -- I'm totally jealous of your boobs and can't wait to buy my own, lol, so i mean it in the nicest way possible!)
But I think you should buy that dress whether you wear it for graduation or not.

It's so cute!