If they had the guest list beforehand, they had a responsibility to inform you of this issue before the trip. Putting you on the spot forced you into making a crappy decision either way you went with it.
I personally would have yanked all the students out and explained why and exactly why you were all leaving to someone in authority if this couldn't be cleared up, further that it was going to go up on the Internet, but my personal bias stems from years of being excluded from things for no obvious reason as a kid.
Think about what benefit that would give the other class members, though. None. All they would be feeling is that they missed a lecture they wanted to be at and should be at.
It's sad for the student who could not attend, that is obvious. That's Yelp's fault. But these kids aren't kids. If they wanted to stand on principle and leave the presentation, they could make that call. But the decision to pull the whole class from the conference shouldn't be up to the teacher of a college class populated by adults.
Throw a shitstorm at Yelp, yes, but if I were in that class and the teacher told me to get back on the bus, I'm under no obligation to listen to him. And I likely would have stayed, almost equally as upset at the poor behavior of the instructor. The way Chris handled the situation was about as good as you can get under the unfortunate circumstances.
I personally would have yanked all the students out and explained why and exactly why you were all leaving to someone in authority if this couldn't be cleared up, further that it was going to go up on the Internet, but my personal bias stems from years of being excluded from things for no obvious reason as a kid.