A lot more people are held back then skip a grade, but both sets make up a pretty small minority of the student body, less than 10% IIRC but I'd take that value with a grain of salt, pretty much everyone who drops out has done so prior to the prom and a pretty heavy correlation probably exists between those who fail a grade and drop out.

Most places in the US have September first as the cutoff to turn five to enroll in kindergarten, though I swung in at 4 with a mid-September birthday and that's not too unusual. However, this changes the statistics, since by and large anyone with a birthday between april and september will be 17 at their prom, or about five months, 41% and a bit less since obviously some people with april birthdays will have them before the prom, so call it 38%, or 62% who will attend as 18 year olds, although in point of fact the birth rate in the US tends to be noticeably higher in July-September, peaking in September, but as that is over the break-date it probably averages out, more or less. The big drop though is that only one of the people attending has to be a senior, and they often bring juniors as dates and more rarely but not uncommonly people who graduated the previous year. If we guessed about 1-in-4 brought a junior as a date, that'd drop it to about 50% of attendees being 18. So probably about half, not a quarter, either way making it a significant problem since a big chunk of the attendees will be underage.