EDIT: Sorry, this came off a bit too personally. I'm a teacher too and I got angry when I read this story. I'll just say more generically that it is easily possible (and justifiable) for a student to feel alienated after a situation like this, especially if they feel like their educator didn't stand up for them. Whether or not "standing up for them" involves canceling the entire trip is a contextual issue, and I really don't know if it would have been a good idea in this case.
Imagine you're a student in that class; you're stoked to see this talk, but one of your classmates can't get in. You feel terrible but you've been looking forward to this. If the teacher decides to "stick it to the man", you and 8 other of your classmates miss out and possibly become bitter towards the excluded one because of Yelp's security measures.
His having to direct her to the nearest transit might seem cold, but he was responsible for all of those students, and he was trying to offer them an experience they would grow from. I would have been incredibly bummed if I were her, but it wasn't anyone in her group's fault and they shouldn't have missed an opportunity to learn because the teacher decided he wanted to take some sort of personal, "Yeah, well I'll show you!" stand.
"Why vote, I'm just 1 vote. It's not going to make any difference."
And you said you're not important enough that anyone would have noticed? So all the 'unimportant' people, when faced with difficult choices, should just roll over and go with the flow? (Not saying you made the wrong/right choice, but your reasoning is off.)
While you had to weigh a group of students' benefit from the talk against 1 missing out on some talk, this also could've been an opportunity for you to teach your students to a different kind of lesson.
I hate to say it but leaving a student alone waiting is pretty shitty.
That's a pretty shitty experience, to be the only one who can't get in and have to walk away alone. A little solidarity would have gone a long way.
Edit: read your comment in a separate post. It would have been worthwhile not to abridge all the context out of your story.
Other venues are much more reasonable about this sort of thing. I have a friend who is banned from all Microsoft buildings, including non-Microsoft events hosted by Microsoft. He was informed clearly in writing, and knew the reason for his being banned. That makes Microsoft-operated venues more appealing (on at least this axis) than Yelp ones, since organizers of an event can't be sure that attendees will actually be able to enter. In the Microsoft case, at least a person seemingly knows whether they can attend.
Here they are practically claiming she is a terrorist. The principle is the same.
By sending her out you validate their discrimination, which is based on falsehoods and lies.
What if [..]
We can all sketch awful situations in which the appropriate course of action is obvious. This is not one of those cases. By sending her out you validate their discrimination [..]
No, you acknowledge they have authority in the situation.