I just did this two years ago. We live in the SFbay region and were shopping for an '8th grade dance' in May.
A fashionable 13 years old aquaintance recommended that we shop at :
Betsy Johnson (that year: too trendy for us - not elegant)
Nordstrom - we were in a 'special occasion' department - not juniors- and I don't quite remember what it was called
Hollister (never made it)
and you can extrapolate to Macy's etc
The aquaintance gave instructions/suggestions: strapless, short dress and that turned out to be the fashion for their class dance. - semi-formal - mini-prom. Thank goodness for her, I was clueless - what to get, where to shop.
We were fortunate to shop as two mother-daughter pairs and we moms agreed that this made everything much easier.
For my daughter we found a light blue dress at Jessica McClintock and her friend found a dark purple, strapless, short dress at Nordstrom.
The IT's dress had a strapless heart neckline (as opposed to straight across) and three diaganal swags/draping of cloth that balanced and her figure and gave her nice gentle curves. The skinny hourglass had a dress like a Dagas ballerina sculpture with a straight across strapless cut with skinny spaghetti straps (that made quite a difference)
The girls went ahead to MAC and we moms came later and approved the eye liner, mascara and lip color & gloss they chose with the sales reps help. They looked enhanced without looking too made up. Neither girl had really worn make-up so this was a real venture for them.
The shoes threw me at first. Our girls weren't really comfortable in heels - no experience & still teen klutzy. We chose: gold strapped flat sandals for my daughter, kitten heeled strapped sandals for her friend and another friend chose slightly higher but chunky heels for the stability.
Minimal jewelry. Maybe a short delicate pendant.
No purse - well... maybe a friend had a purse and cell phone, but not her.
I hope you have a good experience with this.