The making of this movie was the topic of a NPR Planet Money Episode (Episode 650 — The Scariest Thing in Hollywood — which discusses the economics of the low budget Blumehouse Production.
http://www.npr.org/sections/money/2015/09/11/439632877/episode-650-the-scariest-thing-in-hollywood
Ironically the reason there was no-one at the dance, or extra, was budgetary concern. There is actually a whole part toward the end where the director, Rob Cohen, discusses how the car-chase scene forced the movie to go over the 4.2 million budget and what he did to get the $24,000 to get the fake head to do the epicene in the eye shot.