This idea I had heard of for people who are on a budget when getting married and I think it would work well for graduations. If the budget for a photographer is not there or in addition to a professional photographer, simply provide a bowl of disposable camera's in a general area and encourage students, staff, guests etc... to take photos throughout the even, at the end have a staff member, or student services or a depository of some type available at all the exits to collect cameras. The developed pictures are great with perspectives and shots that are natural and unscripted and can also serve well for both retention and admissions. Well worth a try.