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? A ban on shunning is not enforceable in the way a ban on physical violence is.
Your initial point was that shunning was part of a right to free association. I disagree. MegMeg pointed out that people in the are expected to show common courtesy and that this behavior should be expected of students in schools. I agree with her. No one has to be friends with Jim, but refusing to be polite at lunch and shunning him is wrong, too.
I was on the receiving end of this treatment when I was 16. My family moved to a part of the US that was very different from where I'd spent all my school years before that. A lot of the kids in my new school decided I was weird.
Note: people at my old school didn't call me weird, and I had a lot of friends there. I was just different in the new place, and a lot of the kids had trouble with that (though I didn't understand this at the time).
My first couple weeks in the new school were hell, primarily because I couldn't find a place to eat lunch. Everyone sat in groups in the same seats every day (custom, not dictate), which made the situation even more difficult. My sole new friends (n=2) ate during a different period. No one else wanted me around, and they didn't have to say "GO AWAY" to make the point. I dreaded school and dreaded lunch even more. Mostly, I ended up sitting alone in a hallway lined with benches rather than face the rejection of everyone else. It was awful.
Finally, a kid in the senior class invited me to sit with his group of friends. Later, I realized that he knew what was going on and was going out of his way to help me fit in. I became pretty friendly with a couple people in that group of 8 or so, but we mostly just made polite conversation at lunch (as MegMeg noted). It was a huge, huge, positive development for my mental health that year. And I am speaking as a stubborn, outgoing person who is not easily affected by whether others like me or not. That I am this way---and yet felt so miserable---says a lot about the power of shunning. And it must be even harder for people who are shy and/or more sensitive than I am about things like that.
I realize that sometimes our society goes overboard about certain ideas or practices, but I don't think that bullying is one of them. It's a serious and semi-invisible problem, and allowing children and teachers to get away with it endorses it and encourages it to continue in adulthood.
The problem, IMO, is that people rely too much on hyper-reacting to instances of bullying once they get out of hand instead of setting meaningful expectations for appropriate behavior on the first day of kindergarten (and continuing thereafter). A little compassion and and a little courtesy go a long way in pre-empting the problem to begin with.
Added: our daycare provider used to handle this kind of thing exquisitely. If a kid said something like "Let's not play with Jim because he's weird" or "If you don't do X, I won't be your friend," she'd jump all over it. The offending kid would be told, "In my house, we don't treat other people like that. We respect others, and if you can't respect other people, go play alone. We want to play with you, but I can't let you play with the kids unless you can treat other people the way you want them to treat you." I wish more people would handle that sort of situation like she does.
