Having the coverage not only protects the DJ but also helps protect the client/guests as well.
Another important key element to looking out for a professional DJ is their music and how they acquire their music. Registered DJs contract with production studios to obtain licensed legal copies of music. This is important to you, the shopper, because playing music at an event requires ASCAP certification (Enables DJs to legally play music at events) and a registered DJ will have that specific certification to play at your venue.
One last key piece -
For school dances, a DJ really should have a sub-woofer and decent lighting. Ask your DJ if they are providing a sub-woofer and lighting. Request a link to where you can see their equipment actively being used at an event.