Last year I went overboard with the pics, but there is another good place for plus sizes, Fashion Bug. They got some nice black dresses for pref night and nice skirts for skit day.
Pref night
http://fashionbug.charmingshoppes.co...D001323&nav=FP
Some of the stuff are age appropriate, others are not.
Like kddani said, find something that flatters your body, not an outfit that will make you look like a potato sack.