I honestly have a boy who was bullied in every school we've tried to put him in, and that's why he's now being homeschooled and loves it. However, there are times when I notice that he's overly aggressive with his brother, and we discipline him accordingly. (Times out, take away his privilidges, and also tell/show him how he SHOULD be treating his brother.) We also insist that he apogolize with sincerity for his behavior. He has his spurts where it's worse than others, but overall he does pretty good with his brother and the kids we have that come over.
However, kids have a way of being mean without realizing that's what they are doing at times. (In some homes the parents are installing it in their kids without meaning to do it.)