It's a parent's responsibilty to protect the child. You have to pick and choose your battles, of course. Ideally, you can demonstrate how to handle conflict effectively and they'll learn from you so that as they mature they'll be able to handle situations themselves.
I've got quite a bit of first-hand experience with this, actually. My older son is briliant -- he gets perfect grades on all subjects, scores in the 99th percentile on all the standardized tests, was reading at a 12th grade level in fourth grade, has a better vocabulary than the president of the United States (OK, so that not a very high bar). He's also outgoing, good looking, assertive, charismatic, culturally savvy, socially fearless. So in many ways he's easy.
But behaviorally, he can be a huge pain in the (neck). I have gotten calls from every day care provider, classroom teacher, principal, gym, music and art teacher he's ever had. Sometimes on a weekly basis. He was supended from school in kindergarten. I never know what advice to give to the teacher about how to handle him, because he's just as difficult at home. (For the past year or so, he's been doing a little better, but he's still very very challenging at home.)
I don't know that I've ever disputed a teacher's complaint, or tried to claim "My son would never do such a thing." Because I know he would. I know he'd be tough to have in class -- a breeze academically, but one of the hardest kids to handle behaviorally. So even if my son has a completely different account of an event -- and he almost always does -- I tend to side with the teacher unless there's really strong evidence to the contrary, and even then I don't usually make any fuss about it. If my son complains, I remind him that in the real world he'll have to work with bosses he doesn't like, and sometimes he has to learn to get along with teachers he doesn't like.
But his teachers have always used discipline methods such as detentions, trips to the principal, isolation, etc. Never anything humiliating. My son would be hard to humiliate anyway; he's extremely thick skinned (one reason he's hard to control). But still. He did start at a new school last year and even
he was a bit nervous. The example of the kid driven to tears on his first day, when he's trying really hard to like his new surroundings and make a good impression on his classmates, still makes me really mad.