When to shut students up: Civility, silencing, and free speech

Teachers sometimes shut students up for the sake of civility. My question is whether silencing for the sake of civility can be morally justified when a student derogates fellow students as members of some widely stigmatized group, and the offending speech is not for any further reason to be deplored, for example, as a personally targeted insult. Exploring possible answers to that question sheds light on a bigger issue: the proper character of 'civility regimes' in educational institutions whenever group stigmatization persists in the social background and impinges seriously on some students' lives. A plausible argument for silencing under the conditions specified is derived from respect for students' equal dignity and the protection of fair educational opportunity. That argument is nonetheless defeated by considerations about the rightful place of intellectual candor in a culture of free speech and the centrality of educational institutions in supporting candor's development.