I can't find it now, but I think this one has been discussed here before; the scenario is awfully familiar.
My opinion: It's not an official school event, so there's absolutely no reason for people to not drink. I would look askance at someone objecting; I'd probably expect them to use a lot of 'thees' and 'thous' and express admiration for Cotton Mather.
I do lots of stuff for youth: I'm a youth soccer referee and I sail as a volunteer with a youth outreach program. Both of those activities are completely dry, which is the right thing. I can guarantee, though, that everyone of legal age there partakes of an 'adult beverage' off hours. (Think about it -- sailors!)
I know quite a few teachers and more than a handful have "fortified" themselves before PT conferences. As long as they aren't smelling of alcohol and are behaving soberly, what they do off the clock isn't relevant.
As a final note, the boys' school has grade-level parent potlucks and other events (without kids) where alcohol is served.