Aaah, the cowardly act of ganging up on one person. This happens quite a lot unfortunately, usually with far more severe consequences. I disagree with beating them up though. I would definitely stop that "fight" when I saw it, but fighting a bunch of kids? I like fights even, unlike them. And I don't know if humiliating them would help, some of them seem immensely frustrated (hence the agression).

Drag them to a boxing gym or something, there they will learn whether they really like fighting (it's different when someone hits back).