"Clash of the Egos"

[DracIV]

No, this isn't a ripoff movie of "Clash of the Titans," but instead a short question on how best to deal with team problems.

When you gather a group of 15 people, many of whom are already part of a seperate heirarchial social structure, what is the best way to forge those people into a team? What do you each think is the best way to overcome the web of social jabs, dominance, and teenage egos to make every person act smart (in judgements), work hard, and rely on teammates?

Background: This is a question dealing with problems on my soccer team where the players are highschoolers of all ages and can't seem to unify. I am hoping that the critical thinkers here can help where my own thoughts and attempts have failed.