Get a DJ at least one month prior to your event. Let the students do the legwork for you in finding a pool of candidates for DJ's. They are usually very aware of DJ's available in the area. After the students have narrowed the selection process to two or three potential DJ's, you can then check references. The references that will be most helpful are those of other schools.
School dances have different needs than private parties or other events.
This way you can check about things that concern you as your school's representative. A bad DJ can ruin a dance, so take the time necessary to make a well-informed choice. Most schools have a contract with a particular photographer to take couple and group photographs at their dances. Talk to your photographer well in advance to allow for adequate coverage for pictures. Some schools have students that like a lot of pictures taken at every dance, while others don't.