If teachers witness one student being picked on verbally, of course they should stop it. And there's not a damned thing they can do about the content of the students' thoughts. At some level, the responsibility for policing the students' mouths must fall back on the parents. And eventually, on the students themselves. At some level, at some point, the students must take responsibility for their own action.
People throw off their parents' teachings.
For that matter, people absorb things their parents never taught. At some time in their youth, what young men and women choose to do comes down to their own personal responsibility. Certainly when it crosses the line into physically attacking another. That said, I believe that the schools have an absolute duty to protect their students from physical harm, physical harrassment. To be sure, not all of the incidents listed above occurred on school property, or under school authority.