I think the whole thing would be made better if "Prom" was changed to "Leavers' Dance".
OP - it sounds like your daughter had multiple opportunities to sort herself out, and she hasn't, so banning her from the Prom is perfectly acceptable.
As for the timing - yes, it was unfortunate, but you have said that the school has accepted that, and as I said in an earlier post, most students in most schools are registered and put into the exam room well before the exam so I find it unlikely that your daughter was told "5 mintues before the exam started". Presumably, that information came from her anyway, so it may or may not be sprinkled with a little bit of teenage exaggeration...
I still don't think I get your point - do you just want people to sympathise, or are you after some way to get back at the school - you do seem to think that the news about the Prom exclusion will perhaps be acceptable reason to complain if your daughter doesn't perform well in her exams, though I may be reading too much into that.
I am a teacher. We have a prom. It's lovely. The students organise it themselves, through a Prom committee, which is comprised of volunteer students, some sixth formers, and a couple of Heads of House (we don't have Year Heads) I teach in a fairly low-achieving school, and we do have behaviour issues, but the Prom is usually a fabulous event. Parents do not get involved in the organisation, and I have never heard of any pressure on the kids to spend huge amounts of money. We certainly don't advocate the "date" side of it - most turn up in big friendship groups. In the main, I approve. Even before the advent of High School Musical, and the big push for Proms which came before that, many schools had an end-of-5th-Year disco or dance - we certainly did when I went to school. It's a nice opportunity for the children to celebrate the end of the year, and I think it's parents, and kids themselves, in certain places, who do the pressurising. It's not universal.