Keep in mind the neck on your graduation gown is actually a little low. So the shirt you’re going to wear underneath is going to show. So, for men, you should wear a collared dress shirt. Adding a tie could be a nice touch, but you don’t have to.
For women, because of the lower neckline, a scoop-neck top with a modest cut or even a collared blouse is suitable. Or the much more popular attire – a dress. Not too short – you will probably take pictures with your gown open before and after the ceremony.