Thanks for each of the replies and tips. I welcome any others.
The additional problem for me on the occasion I described in my opening post is the fact that I am representing my school at this conference, so 'professional networking' is part of the deal. So an early escape route was not an option, sadly. However, for more normal social activities, the early escape plan looks like a good tactic. I'll have to start inventing some plausible excuses. Thanks again folks.