The only folding knife I would trust for a stabbing is a butterfly(balisong) knife. Any knife would probably be fine for slashing, but I don't think that would be effective enough against a gunman. I figure your best chance with a knife is to wait next to the door if you hear him coming, and go for the neck as soon as he walks in. As someone pointed out, a regular pen would probably work, though I imagine it might be hard to get a secure grip for the amount of thrust needed.

This type of massacre is so rare that I wouldn't carry a butterfly just for that purpose if I was still in college, but I've thought about getting one if I move to a big city, for muggers and such.