The anti-bullying crusaders never talk about free speech or the right to free association (which includes "shunning").
Expanding on geofizz and JonLaw, a good analogy is to the workplace. You don't have to like your co-workers, you don't have to be friends with them, you don't have to agree with their values. But you do owe them basic common courtesy. The same should be the standard in schools (or any environment where there is a societal expectation for people to be together and interact, which they cannot reasonably escape from).
Shunning should be reserved for communities where people have chosen to come together with a shared belief system. It has no place in public organizations in a pluralistic society.
