The length really depends on where it hits when you try it on so it's hard to say, but they do provide a measurement of the length of the dress to give you an idea. I have a longer torso and long legs so for me this dress would be short lol it just depends on you. The bust seems a little too mature for your age. Another thing is the lack of straps. If you're going to be dancing at this dance... don't raise your arms! Overall the dress is cute, but it seems more like a party dress for someone older. Not something I would wear to go dancing.
I really like it ! I think it's elegant but at the same time sexy ! If you are tall you could wear ballerina shoes (that makes everything more casual). If not a peep toed shoe with a heal would look great under it (to make it more casual wear your hair down). Just accessorize it well...it's how you accessorize that really dresses you up or down. Don't be afraid to look too mature : just cause your young doesn't mean you cannot be stylish. Trust me on this-I'm italian :)