Check with the photographer to make sure that your classmates will have the necessary ordering and payment information. If you use a photographer you're unfamiliar with, ask where the portraits are printed. Companies with their own printing labs usually have more control over quality, selection and delivery time. Ask what package selections are available and what type of information is provided to your classmates about the photographs. This is also a good time to ask about rebates to the school to help defray the cost of the event.
Rebates are standard practice for most school photographers.
If you make your own background, consult your photographer about the appropriate size and materials. You don't want the background to detract from your classmates and their formalwear. Be sure to get the delivery time of photographs in writing. Professional photographers know what it takes to make your event a success. They realize that this is one of the most important nights in the lives of you and your classmates.