Name and author of a short story, about a teen who murders his family?

I thought it was S. King at one time....similiar to the story Rage. The story was about a teacher I believe, talking to a class, about a teen who had massacred his whole family, for no apparent reason, and then went on a rampage with the only survivor, an infant, in the truck beside him. And at the end, you realize the teacher is the surviving baby....It struck me as a creepy story years ago when I read it, before columbine and all the mass rage killings that followed, and I've been trying to think of the name of it and who the author was....