There are a number of ways to lengthen too-short prom dresses. All of the following ideas will work for both short and long prom dresses.
Slip extenders are commonly worn under too-short skirts to add the appearance of length.
Dresses with pickup points (bunched skirts)
Dresses with pickup points – bunched skirts – are popular these days. On these dresses, the outer skirts is bunched up and tacked to an inner lining to create poufs of fabric all over. You can lengthen a short prom dress that has pickup points simply by snipping some or all of the pickup points. Usually, a pickup point takes up 4-6″ of material, so sometimes a dress with pickup points can be lengthened significantly.
Some short dresses are balloon-style – they have an overlayer that is gathered at the bottom, looped back up inside, and attached to a slim lining. Balloon-style dresses can typically be lengthened 4-6″ just by detaching the overlayer from the lining, then letting it hang straight down.
Dresses with a tulle band
A lot of short prom dresses have an underlayer with a band of gathered tulle at the bottom which is designed to peek out from under the dress. You can make a dress longer by adding a thickly-gathered band of tulle behind the existing tulle. Most dresses have a lining that you can attach tulle to even if it doesn’t already have any.