Consider the ability to dry clean what you wear. A little fact that they don't always tell you is that many gowns bleed color if they get wet or even hot. So don't wear something you care about too much in case it gets gown dye on it. It may not come out even if you do dry clean your outfit, and big stains look bad in pictures.
Consider the weather. My high school graduation was unbelievably hot. My sister's college graduation was so cold that my whole family was huddling under blankets with coats on, and we were all wearing long sleeves to begin with. Be sure to watch the weather reports the days ahead of the ceremony. It may drastically change what you plan on wearing.
See if your school has a graduation dress code. Some schools might not care what you wear while others may have strict dress codes. Before you make any decisions, consult your school's policy.