Rows
Position the subjects in either three rows of four or two rows of six. The subjects in the back should be positioned in between the subjects in the front so that at a minimum, the heads of the subjects from the back will be visible. Place taller people in the back row. Those in the front row can kneel to make any rows behind them more visible.
Rule of Triangles
Going by the "rule of triangles" is another way to photograph groups. Though this is usually better for smaller groups, it can also be used for larger groups. You can form the triangle with three people in the back, two in the middle and one in the front, or you could put six in the back, four in the middle and two in the front.
Shoot From Above
One interesting shot angle used for photographing large groups is shooting from above. Find a location outside where you can take the photo. Find a playground, roof, tall ladder, or other object that will elevate you enough for the photo. Have your subjects look up at the camera. The way that rows are situated is not overly important for a shot like this, since all subjects will be more visible from above.
Multiple Poses
Have the subjects in whatever pose they wish. It is very difficult to have everyone looking in the same direction with all of their eyes open at the same time. To alleviate this problem, give the subjects the freedom to choose whatever pose they wish and to interact with others in the photo. Encourage them to act silly. Sometimes these bring out the best in people, creating exciting photographs.