There are several factors at work here.
The first is that while the students should behave in a more adult fashion, the teachers need to recognize it's the students' rights to not behave in an adult fashion. Prom is an event held for seniors (yes, underclassmen do attend as guests. This is an important point, but the event is not held for them). In May, a senior is a high school graduate minus one week. In fact, non-graduates are usually not permitted to attend prom. The fact is, these people are for all intents and purposes adults who shortly will be doing the same dances at college parties only not in formal wear - except at Greek formals when it'll be even worse. If you want to stop sexually suggestive dancing, the time to intervene is well before senior prom - I strongly support Dancing Classrooms both as an alternative and as a way to give people an alternative style of dancing - but once the prom rolls around you have to trust the students to do the right thing and live with the consequences of a base, tasteless affair if they don't. As for underclassmen, it's for their parents to decide if they're comfortable letting younger sons and daughters attend an event knowing that sexualized dancing is likely to take place.
Then there's the outrageous sums of money aspect. I don't know who to blame for this. We live in a culture that not only maintains but encourages a princess mentality. True, the teenage girls buy into it, but what about their parents who support it, what about their boyfriends who respond to it, what about their peers who pressure them into it? I rented a tuxedo for my prom (the last time I ever did. For what it cost me to rent a tux, I purchased a phenomenal used tux and got it perfectly tailored), while my date borrowed a vintage kimono and obi. With an orchid corsage to match the kimono's print, I assure you we were a stunning couple. For transportation we carpooled with a friend who restores vintage cars and had a great time. The fact is, however, because satisfaction can't be quantified emphasis is put on cost end effort as a standard of social value. People aren't taught how to have a good experience, so they buy into the trappings - literally.