One way to pose a large group of people for a photo is by having everyone sit on bleachers, or tiered rows of benches. If your class reunion is being held at your old school, chances are you will have bleachers available to you. Due to the tiered seating, each person's face will be visible in the photo, regardless of where each person sits.
Photos taken from a bird's eye view work well for group photos of any size. In fact, many wedding photographers take a bird's eye view photo of all the guests at a wedding. To pose the class for this photo, have everyone stand in a tight group and turn their faces upward. For an added effect, you can ask everyone to put one hand on the shoulder of the person in front of them, or hold their hands above their heads and wave. The person taking the photo must get high enough that he can hold the camera above the group, either by standing on a ladder, the top bench of a bleacher, a balcony or anything else that will give him the necessary height.
A large group of people organized into various triangles creates a visually appealing photo. To pose the group, tell everyone to get into groups of three, then have each group stand with one person in the front and the other two people on either side, slightly behind the first person. Make sure that the people in each triangle are close together, and scatter each triangle so that they are equally spaced. The triangles can either be all in one row or various rows. You can also have some triangles of people sitting and/or kneeling.
This is a common pose for large groups of people because it allows for each person's face to be seen while adding dimension to the photo. Arrange the group by having a row of people standing in the back, the a row of people kneeling in the middle, and finally, a group of people sitting in the front. You can have the entire group doing the same things with their hands or you can have each row doing something different. For example, the row of people sitting can rest their hands on their knees, the row of people kneeling can have their hands on their hips and the row of people standing can have their hands clasped in front of them.