A small group of teens are going to or leaving prom nite and they get hit by a truck transporting chemicals or something and it brings them back as zombies. I think it's a late 80S early 90s film and I remeber a couple of the deaths in the movie. In one scene they spit in a cop's face and break his neck. In another they kill a mechanic by pumping him full of oil. Also, at the end of the movie they push two people into a woodchipper, and the girl that was killed was wearing a dress.
Seems it's Night Life from 1989. All the details are there (teenagers, prom, chemical truck, wood chipper, overpumping someone to death). Beware - graphic images inside: http://www.the-losthighway.com/2013/02/17/night-life-1989/