Drinking at eighteen is certainly allowed (actually, there is some drinking with meals before eighteen, in sixth forms) s
I suppose if you are already paying international fees at St Andrews, the cost of a few balls are not much. I know at Oxford, some people find them fairly expensive (but usually not more than 100 pounds or so, which is admittedly not cheap).
Wow. I think her Hall ball is going to run around 35 pounds. I dunno. Social expenses are on her. She will reuse her junior and senior prom dresses and swap with girls in her hall. In fact her senior prom shoes have already been to a ball without her. 
I would say formal attendance would be US Students in Greek system> UK students> US students not in Greek system
I would say Oxbridge > US Greek > other UK > US.
Oxbridge balls can cost over £200 if you also do the dinner, though most are nearer the £100 mark. They do both black and white tie balls, and most of the 38 colleges have a ball most years (white tie tends to be the triennial ball, not the annual ball), so if you are interested you can go to a lot of balls. The bigger colleges really splash out with up-and-coming bands(ie, the students have heard of them, even if the parents haven’t- Coldplay is an example), bumper cars and other fair-type activities, silent discos, fireworks, casinos, fancy booze arrangements, etc.
This is a story that the Daily Mail newspaper did about the triennial ball at Trinity College (a medium sized Oxford college) in June (with a little clip of the silent disco, which is always odd to watch!): http://www.dailymail.co.uk/news/article-4634994/Revellers-return-200-ticket-Oxford-college-ball.html.
Quick update on Class of 2021 kid.
Great first semester. Grades high enough for Dean’s List. (which is only for GPA for the entire year, so her goal is to do as well second semester and make it official for the year). Being on a whole different grading scale and having ~70 percent of each module grade based on the final was a little anxiety producing at first but this has reassured her that she can do it. Some friends did not fare so well, one stopped going to some of his classes because he didn’t think they were that hard, and so he has to resit an exam or two. In that regards, parental advice is the same world wide- go to the lectures, keep up, review notes, and so on!
She did get tickets to the Christmas Ball and had a great time but that’s the only formal event she went to in the fall aside from formal hall dinners, which are about once a month. She went up to Edinburgh a couple of times for shopping and clubbing, but in general has found that the social scene in St. Andrews itself is sufficient for her needs. She’s a bit of a homebody, likes to workout, cook at home and study, with just some partying thrown in.
Glad it has gone so well, @VickiSoCal! & thanks for the update.
@VickiSoCal which hall did she live in? Also Congrats on her grades that is awesome. My son did well as too but does not sound like quite as well as your daughter. His proudest academic point of the year was a 20 on his Statistic Exam! Good thing though as another one was rough! My son lived in McIntosh and LOVED it! Next year he is in an apartment.
She was in St. Regulus. Second semester classes were harder and grades just a bit lower but still US A minus average.