Warriors go head first,
Rangers stay behind them,
Elementalists, monks, necros and mesmers stay at the back, just doing as much damage or healing as much as possible.
through my observations i noticed what could be the priority people acquire their targets. it is something like this: monks (they heal), necros and mesmers (they hex), elementalists, rangers and warriors. so if you go to an arena and are in a team with 3 warriors be sure you'll be targeted and killed first.