Where can you donate a prom dress?

well at my school they have this prom dress sell or something like that were they get a bunch of dresses and sell them for cheeper then what you bought them. after the dress is sold then i think that you and the people that helped you sell the dress get the money i know that you get the money but i dont know what they get. but this way you make some extra money and you get rid of your prom dress