This is where all the cool kids hung out in Junior High. The closer to the back of the bus one is usually signifies one's social status. The losers(i.e. nerds, retards, smelly kids, etc.) sat in the front, while the cool kids(i.e. jocks, rich kids, sluts, etc.) sat in the back.
"Dude, look at that slut Sally making out with Billy in the back of the bus!"