What are the social norms associated with stealing free coffee?
The big difference here is simply whether the meeting is over or not, i.e. whether it would have to be thrown away or not. If those who are supposed to consume it (for which, as others have pointed out, someone paid) clearly have no intent of doing so (because they left), then I think it is fine to consume it. 

In fact, in that case, you would even do them a favor because noone has to clean it up. But otherwise, it's stealing. Another indication for this is the group size: With about 100 students, there clearly wouldn't be enough if everyone took some coffee.