It really depends on how I feel.
I once struck up a conversation with a complete stranger because they were wearing a band shirt I liked. I was warm, made jokes, and actually laughed and smiled. This was in a group of a girl I knew as a loose acquaintance a few years ago, and 2 complete strangers. I think I came off as quite extroverted in this scenario. Looking back, I'm not quite sure how I did it, or why.
In general, I'm aloof, quiet, and seemingly either a very angry or sad person, apparently.
I don't smile when I talk to people and I either stare at someone by accident or look away. Someone once told me I scared them.
Whenever I see someone I used to know for the first time in a few years, they seem shocked at my seemingly confident, straightforward, opinionated and "creepy" appearance.
In a situation outside of school I come off as extroverted and strange, I think. If it's someone I think I'll never see again I act like I know them already.
If I'm somewhere I don't want to be, I act antisocial and retreat into my own world, either by reading/studying or just ignoring them.