Warning: The following thought experiment does not adhere to the status quo. No need to fire up the flamethrower. If a difference of opinion may be upsetting, scroll on past.

OK. I would have reacted exactly the same as everyone else in the classroom did.

Not knowing what the girl may have done or said to the teacher before class to deserve banishment, not knowing if, e.g., she had made sexual advances toward him or cheated on an assignment or exam (sadly, daily occurrences in schools throughout the land), we have to assume it is a private matter, none of our concern, and not an automatic miscarriage of justice. We conclude there must be a good reason for the authority figure, if and only if he was observed by all to be sane and rational prior to the incident, to make her leave; and we politely mind our own business.

A classroom is somewhat like a traffic stop: it sn't a level playing field for communication. There is a clear imbalance in the power dynamic. That dynamic is strongly in favor of the teacher. To keep order in a classroom this is necessary; but it means teachers get to say and do things that students definitely do not get to say and do. Teachers get to challenge students in ways that would be wholly inappropriate and disruptive for students to do to teachers.

Dude taught a great lesson there that they'll all remember. The soul of his lesson is totally excellent: Speak up when there is CLEAR injustice. I just think it'd have been better if he'd given some bogus reason for her banishment, so 100% of the students would not have assumed it must be legit and later been embarrassed en masse by the teacher just for being respectful of him. They did what at the time seemed reasonable and correct, given their lack of adequate information to stage a protest upon. Rising up and storming out on him would have looked like a knee-jerk reaction based on no knowledge of the situation—especially given that the girl HERSELF didn't even bother to protest at all. She just muttered "I don't understand" and walked out. This could easily be interpreted by her fellow students as guilt: If this is really such grand injustice, why can't SHE be arsed to voice even the mildest objection to it? Her quietly slipping out supports the suggestion that she may know she's done something wrong. This isn't assuming she has; the point is that no one knows, so staying out of it isn't a bad move. You aren't accepting injustice in a situation if it isn't clear that any exists.

This teacher should be prepared for students now to interrupt, question or attack anything he might say from this point forward. Fool me once,....

Sorry for seeing it differently but that's how it is IMHO