Just tell them that you don't want to go...however, I imagine they would love to attend the ceremony and see you graduate. THAT is one of the most proud moments a parent can cherish. This is a huge step in the right of passage for an adolescent AND parents.
By the way .. congratulations..That is an accomplishment worth celebrating.
You should do it for them. Graduation ceremonies mean just as much to the parents as they do the students. It gives them memories to hold onto, now that their baby is all grown up, and moving on in life. You may not understand that now, but if you ever have kids of your own, you will understand one day.