AIBU to surmise then that the remaining staff members did have a drink - but in their opinion they weren't drunk - and is this actually acceptable? I don't think any teachers should be drinking on a school trip but now I am doubting myself and looking for opinions please!
I personally have no issues with some of the teachers sharing a drink or two, providing that
(a) they weren't drunk - a bit of loud voices, according to a 13y, doesn't mean the teachers were drunk. Some people are louder than others just naturally, and
(b) at least one member of staff was totally alcohol free each evening, incase of emergencies
Please also bear in mind that whilst those teachers are on the school trip 24/7 there are actually not being paid for any of that extra time. TBH its no wonder so many schools are scrapping so many residential and school trips!
There’s a ratio of teachers to children.
For secondary the legal ratio of adult to child is pretty high. Most, if not all, residential school trips more than cover this, with extras.
Why does it bother you so much OP?
There were alcohol-free teachers around, and the children were not sat with the teachers observing them drink.
TBH my own DD wouldn't have complained about it at all. She'd feel perfectly fine with her teachers having had a drink or two and were chatting and enjoying themselves. She might mention something to me but not in any negative way.
Heck, when DD went on a residential in Y3 and Y4 the parents sent some wine to the teachers to enjoy!