I think it's a late 80S or early 90s film. A small group of teens are going to or leaving prom night and they get hit by a truck transporting chemicals or something and it brings them back as zombies. I remember 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.