My fiance was wondering the same thing. Here is what I found in a little research and friendly advice, (so far): Most schools offer dress codes published on sites and basic suggestions for men and women whether graduating or attending as guests. In the case of guest attire, this is a basic, nondescriptive guideline. For the graduating class, the dress code is often very specific and those not adhering to it may even be forbidden to walk (participate).
Where weather is not prohibitive, (extremely cold or pouring rain) dresses or skirts with heels are appropriate. Shoes: Don't wear flip-flops, sneakers, casual sandals or casual shoes. Flats are perfectly fine, as are open-toe dress sandals/heels, (weather and venue permitting - you don't want to be tip-toeing over damp grass to avoid leaving puncture holes in the perfectly manicured greens - or - (gasp!) staining your pumps).
The skirt or dress: should be "business length" ; wear something that would not be inappropriate for the office, if a little dressier - that's fine. Many networking alliances and mentors in further education will be there, and the photos will last forever. Think: would I wear this to meet his family or to socialize with my own or my partner's boss/colleagues? An A-line or pencil skirt is better than something flared. No ballgowns or flashy cocktail dresses. A baccalaureate ceremony (pre-graduation) requires a more formal attire, and semi-conservative evening wear is appropriate. A good rule of thumb is whether your middle finger reaches your mid thigh skirt/dress hem when your arm is relaxed at your side. There are plenty of options in fabric and style of cut that can make these dresses. Women may also wear slacks (nice business dress pants or dressy pants with a nice blouse).
Do your hair. If you wear it: jewelry/make-up. SEXY is ok, but think respectable, confident and composed. SLEAZY is not. Garner respect, not attention for your clothing; you will look out-of-place and eager for attention