As with the thread about scout trips the other day.
I would have absolutely no problem with this at all, if it is just one or two drinks, taken with food, drank sensibly.
When my DD was in Y3 and Y4 they had a 3 night residential. I know that the teachers shared a bottle of wine in the evening, with their meal. It meant a glass each.
Likewise when DD goes on a day trip, in the holidays, to London, I would have no issue with the teachers having a glass of wine with their evening meal before coming home, should they wish.
The teachers are on duty 24/7 on a school trip. It is a very long day, and not actually a jolly - hard work. If they wish to have a glass of wine or beer whilst eating, or in the evening, I think good on them tbh!
So long as there is a designated first aider and driver available, in case of emergencies, really what is the problem?