How short is too short?

My senior prom (I'm in the short blue dress). Hopefully you don't think my dress is too short or tight 

It's starting to get that time of year: when girls flock to stores to find the perfect prom dress. My favorite part of high school (no matter how much I hated high school) was always getting dressed up for Homecoming and Prom. I was the Homecoming Rep. for my class my junior year, and prom was always so much fun! My school was too small to have our own, so we had a combined prom with all the juniors and seniors from the other high schools in the area at the casino.

Love this color! Always been a fan of royal blue

I love looking at prom dresses, but I never wanted to wear a long one for fear it would be too long and would get stepped on while I'm dancing. Plus since I'm not exactly on the tall side, I would feel lost in such a long dress like the one above. So I usually went with a short dress, but how short is too short?
I recently read an article on Yahoo! called High School Bans Sexy Gowns. I went to a Catholic High School, and we had a dress code we had to follow for dances, and some of these dresses definitely wouldn't be up to par. This one is a personal favorite of mine: honestly how is this considered Prom worthy?

Prom is an event to dress up, not dress to go clubbing. My college recently had their Spring Formal, which unfortunately I couldn't attend due to a major accounting exam that needed a lot of studying in order to pass. I looked through some of the pictures, and while the dresses were very nice, some were really short and tight, and the six-inch stilletos didn't help their case.
Normally I would be all for freedom of expression and show off what you have, but trust me, some people shouldn't show THAT much. There's a fine line as to self-expression and looking trashy. Of course, there are exceptions. Like if you go clubbing you're suppose to look a little sexier. But those really short shorts that look more like underwear I see during the summer, yeah I don't think so.
I'm on the hunt for longer shorts/skirts/dresses for my trip to Africa, since it's scandalous to show anything higher than the knee in Malawi. I wanted to find clothes that I could wear even after my trip that I could see myself actually wearing. When I was shopping yesterday, I was honestly appalled at my lack of selection of long shorts/skirts/dresses that normal people would wear. America, why must us girls dress like sluts?