I accidentally took a dress from the drama club at my school.
How do you know it's from the theatre department?
I thought it was my girlfriends dress but it looks like one I wore during a play we did last week.
Oh, then yeah you should probably give it back as soon as possible. You already know it's not right to steal since you brought it up so you should do your best to return it.
Did you think it was bad to steal?... Me too.
Since it was an accident I'm sure it will be alright if you return it and explain what happened.