I generally try to sit in a corner opposite of the teacher's desk, or in the back corner. I turn my chair slightly towards the front of the room where the teacher teaches and take notes. I lean on the desk. If I'm working on an assignment, I sit centered, facing forward. I start to slouch at the end of class because- well, I don't really know, lol.
If I'm doing math, I sit centered as above. I don't know why. I just do what is comfortable, so I can pay attention better. "Receives browny points".