You may think it's ridiculous, to some girls it's a big deal and prom is something that they look forward to all year. They may find themselves ROFL at something you find important.
WARNING! THE BELOW IS A SMALL RANT!
But the way some of my peers treat it is fairly annoying. They believe it is a huge competition for the most flamboyant "prom asking", and it is a pain having my class time interrupted by people asking others to prom (with it taking more than a half hour for lessons to resume).
This might sound rude, but I have less than five months before graduation, and in the cases of AP classes (where time really is of the essence and the daily agenda is planned to the minute), those continuous prom-related interruptions can make the difference between a 3 and a 4 on the AP exam, or covering the curriculum or being required to self-teach the last three chapters the night before the exam.
Yes, I honestly do not see the point in prom. I actually had a neighbor claim that "those who do not go to prom are absolute losers", but I would much rather use that money towards first-semester expenses or summer sightseeing. Honestly, in fifty years or so, the prom-related stresses will not matter.
I know I am going to get yelled at for this, but part of me really wants to blow up at those who believe every single decision made about prom will affect the rest of their lives. While they whine about a certain dress not being in a color they "need", I am in the hospital fighting for my life and trying to learn how to manage my diabetes. At least they are alive with no life-threatening conditions, they have the opportunity and financial stability to go to prom, and, more often than not, they will have an enjoyable night. Not having a dress in blue will not cause the world to end.
END OF RANT
As for sophomores/freshmen going to prom, I'm surprised it happens at some schools. Though my previous HS was divided into a 9-10 and 11-12 school, underclassmen could not buy a ticket. They only sold tickets at the senior school and also required ID when purchasing. Juniors were only allowed to go to the senior prom if accompanied by a senior date (and even then, that was highly-frowned upon).
For purchasing a dress, a lot of the schools I have heard of do not post the official dress code until March or April, usually around when tickets are being sold. Unless the OP's daughter has older friends who can give a general synopsis of the dress code or can find a dress that will most likely not be forbidden, it might be best to wait until the dress code is posted. From what I have heard from other HS's, dress codes typically incorporate these elements:
Are backs allowed to be exposed?
If there are slits in the dress, how far can they go? (My previous HS only allowed up to the knee)
Are strapless dresses allowed?
If wearing a shorter dress, how long must it be? (I think my school was mid-thigh)
How deep can the neck line be?
Are torso cutouts allowed? Even with a sheer backing to it?
Will shoes be left at the door or worn during the dance? Is there a limit on how high a heel can be?