Try talking to the person in charge of the teachers (there's usually a "Dance Director" for the teachers) or the Studio Manager.

My first competition was in the Fred Astaire system, and one of the studio managers helped me figure out what to wear. Since it was my first time, I just wore a dress that I could dance in -- one that was a tea-length dress that wasn't too bridesmaid-y, and the other was a cocktail dress that I could dance rumba in. I got them both off the rack in Filene's basement