The sky is the limit with that one! Since you mention Silent Auction I think you want broad appeal. I wouldn't do photos because people are vain and might not bid if they don't like how they looked! (I cringe at just about all my old school photos.)
I would try to tap into the emotion felt by the attendees and build on that. Most people who turn up probably liked the school so that's a good start! Did you have a school motto or a teacher who used a particular catchphrase that people may remember? I'm into poetry so might look for a short quote. If you are religious you might find a suitable quote from there. A few words appliqued onto a simple design in school colours could look great.