It is very important to have a list of all the areas that need to be worked on and the materials needed for the job prior to the event. You do not want 100 students showing up on a Saturday and then start figuring out what needs to be done. Prioritize a list similar to this one to determine what needs to get done first and then start on those projects as volunteers show up. Try to have all of the supplies in one central location to make it easier for distribution and to answer the inevitable questions.
Try to take on this project a week or two prior to graduation in order to make the campus beautiful for the ceremony.