What I like to do is go around and lead people near high ledges and push em' right off! Especially at the big ladder on that one bridge going from Vale to Bullworth Town. A few other good places for this are: The empty pool in the gym, the scoreboard on the football field, the top of the hill at the park (the part with no railing), off the dock at the beach clubhouse, off the dumpster in the Auto Shop, down any staircase (no extra damage, but funny as hell), and off one of the lunch tables in the cafeteria during a food fight.
Also, has anyone else noticed that giving people wedgies does a little bit of damage? I know, I play this game too much XP