what to have at a high school reunion  

Decorate with stuff from that erra. What was fun back then? Books, events, famous people, places to go, type of dancing, etc. Things that will ignite their thoughts and bring them back to that time. Have displays of these things and decorate with the same. It could be most anything that would evoke good memories.

I hope you were still able to keep some pictures. It's so cool if you can play a slide presentation of your batch's pictures back in your highschool days! Also, don't make the presentations too long. Music should not be too loud. IT's best that people will be able to hear each other well since they will surely have a lot of things to talk about and to catch up with. Good luck. I hope you'll have fun. We'll be having a highschool reunion in June.