Suppose a group of children never talk to "Jim" at lunch. That is shunning. How can you force them to talk to Jim? How do you punish them?
First, let's be clear about what shunning is. It is more than just choosing not to sit with someone at lunch. It is a systematic practice of refusing to acknowledge a person at all. In the extreme, anyone else who does not participate in the shunning (i.e. who talks to the outcast) is themselves shunned. Shunning is a severe form of social control that is practiced by small religious communities. In fact, that's pretty much the only place it occurs, except for schools.
Second, nobody here has mentioned punishment. The discussion has not been about particular solutions at all; we are debating what is and is not okay. You tried to elevate the practice of shunning in schools to a civil right, and we are disputing that. (Just briefly, it is far more effective for the adults in charge to set in place a variety of practices that encourage a culture of civility, rather than punish individual instances after they happen. So your "how do you punish them?" question is kind of a straw man.)
One final point. You started out by criticizing the public concern about bullying. It seems from your followup comments that what you are actually against is students being persecuted for choosing not to sit with someone at lunch. Let me assure you that this is not happening. There is no public hysteria going on about students choosing whom to be friends with. The current focus on bullying is about physical violence and prolonged and extreme verbal abuse.
