Have old pictures from the yearbook blown up to poster size. Mount them on cardboard or foam core and place them around the room. Many copy stores can blow-up black and white and color photos to 11"x17".

Take old pictures, sections of the senior pictures from the yearbook work well, and photo copy them. Laminate them and use them for placemats.
Decorate the room similar to the decorations used at the Senior Prom. Although decorating for the prom probably took a lot of time and energy, for the reunion, scale it down. If your theme was Stairway to Heaven, rent a set of stairs, cover them in white, and place snow batting or clouds at the base. Post a large sign with the theme – Stairway to Heaven.

Decorate the tables with products from the past. If it is your 20th reunion, select items popular when you were in school. Buttons from political elections, magazines, books, records, you name it, it will get people talking of the good old days.