Back when I was still in school, it was always pencil. You can't erase pen marks (unless you carry white out with you everywhere!)... But for official documents, I always use pen!
I avoid writing in pencil as much as possible. I hate how light the markings are, and honestly, I can't stand the sound of pencil on paper. It bothers me.
Pens are more expressive (in my opinion), and I like being forced to deal with my mistakes. Or something. I just like the dark ink.