I've taken several school trips abroad and I would be seriously unimpressed with what @iwasjustwonderingreally describes. Ski trips have the reputation for being the worst for this sort of behaviour (among both teachers and students).
As a pupil myself in the 90s, I went on ski trips and we were drinking every night from Y9/10. The teachers let us go out in the evenings knowing this would be the case, and as long as we came back on time and didn't wobble too much, they turned a blind eye. They were pretty smashed themselves, every night. I know loads of staff were desperate to get on these trips as they were such a great time for the teachers! Most students drank and there was definitely some sexual activity and drug use on these trips. And this was among my group of friends who would generally have been described as nice kids who would never do xyz. The teachers obviously didn't know about the drugs but they certainly knew about the drinking. There was definitely a sort of 'don't ask, don't tell' policy!
As a teacher myself, I refused to take senior school ski trips because I knew far too much about what goes on. I did go on a junior school one. Some staff had a drink but they certainly weren't getting wasted, and 2 people were always on call and expected to be sober.
As a trip leader, I've always been really funny about staff drinking at all on trips (or in any other in loco parentis situation e.g. boarding). I know lots of boarding staff who drink once they've finished duty for the evening, some to excess, but if a kid wakes up in the night poorly or there's any sort of emergency, you do not want the staff onsite to have had even a drop. If something goes terribly wrong, even one glass of wine would be seen as a possible contributor to a poor decision or slow reaction to something.
Knowing the kinds of things kids get up to on ski trips, I would want to be fully sober when in charge of them!!!
And in response to the original query, teachers definitely don't pay for anything on a school trip. I've been to multiple different countries and had a fabulous time, but yes it is work and also a huge responsibility.
I'm very surprised that drunk teachers in charge of or in the company of students would fly these days...