I have not done much research on the history of (high school) proms and formals, but if I had to guess I would assume they originally began or were done in a era/culture where couples married early (i.e. around 18 years old) and the prom facilitated the dating/courting process. Like the hedonistic and antiChristian craze over sports, the tradition of going to the prom has likewise devolved into something very unnatural and even sinful. Proms encapsulate everything false about what love and responsibility is supposed to entail.
No doubt, the leading fuel behind prom is the hype, a totally hollow experience packaged in dazzling colors. The source
of this hype is two-fold: First, it is the parents (and "teachers"),
many whom never grew up and want to vicariously live their life through
their children; otherwise, the parents (and "teachers") are stuck in a
fantasy of what they "experienced at that age," often in a time when
high school was the apex of their education and college was limited. But
everyone can readily admit that it's wrong to try and live out your
childhood dreams in such a selfish way as to vicariously do it through
your children. And since going to college is now the norm, high school
is no longer the "academic" high point, but a drop in the bucket which
college students look back at and laugh at the immaturity of it all.
Second, since prom generates a lot of money, various businesses like to
promote it, just like other fake holidays (e.g. mother's day), which
mark the materialization and secularization of true Christian holidays.
These two factors directly feed into the hype that causes the
guilt-tripping of all students into rushing to find a "date" for prom,
while stigmatizing those students who are too dumb, fat, or unpopular,
and cannot find a date to go. This hype turns everything into a
popularity contest and totally eclipses the academic nature of school.
Now, instead of aiming for good grades, what matters is how popular one
is. And those who are in the top tier of popularity fight bitterly over
who will be 'crowned', quite often setting the 'royalty' up for great
disappointment later in life when the celebrity status fades. In the course of this battle, students fight to outspend and out do one another, all to avoid looking bad that night.
Next, it is no secret that proms are obscenely expensive, averaging a few hundreds of dollars minimum, but can easily get up to $1,000 for the more affluent. This kind of spending is very irresponsible and teaches children all the wrong things about the value of work and real life. Such money should go towards paying for legitimate needs, since such amounts can cover the monthly allotment for food or rent for the whole family. No sane person can deny this. And in terms of materialism, often the dress is worn but once and shelved indefinitely, while the transportation, venue, and other needs are grossly over priced. Some might object that it is possible to not 'over spend' on one's prom, keeping the costs at a minimum. While that is true, it goes against the 'splurge' mentality that makes modern day proms and formals what they are. In other words, is a split-personality disorder that desires to to engage the world while remaining free from it.
Finally, and this is the most serious of all reasons, is the issue of sexual purity. This is the most indefensible of all the problems. Though premarital sex is already rampant in both public and "Catholic" schools, modern culture and movies have made having sex the defining mark of an "ideal" prom night. Of course, not all teens go that far, but a significant percentage have, and that's a cause for concern. But even if not all teens go that far, there is still an expectation to get close and even kiss, which is still out of place for teens not ready for marriage (since it sends the body the wrong signals). And this is where the root of the entire problem of prom rests: dating is now totally stripped of any connection to marriage, the same way contraception strips sex of any connection to children. Dating is now just as recreational as sex is, neither of them have a future fulfillment in something greater. This is why any Catholic parent worth anything should forbid dating in high school, because it sends their own child the wrong message: you are encouraged to get close to someone of the opposite gender, go through all the motions of dating, but have no intention to marry them. The great majority of teen students who date have no intention of marriage, and most see the relationship as discontinuing as one leaves for college. This means the child is encouraged to invest their time, emotions, even physicality, all into a moment that has no future in sight. Instead, parents should teach their kids that dating is for the purpose of finding a spouse, otherwise you're wasting each other's time (and possibly the near occasion of sin). And the best chance of finding a potential (Catholic) spouse - if they have not first discerned a religious vocation - is to find or help form a parish young adult group (i.e. 18 years and older), or if at college to find a Newman Center on campus.
Of course, the list could go on and on, from the provocative dancing, immodest dressing, lewd "music", homosexual student couples, binge drinking, etc, etc, but you get the point. The whole mentality behind prom is that of following the crowd, which is the most foolish and unChristian approach to life a parent or child can have. Any mature individual should know that teens are very immature intellectually and emotionally, in short, they're stupid, and that's precisely why being a firm parent (rather than "a buddy") to them is essential.