Harry Potter costume if you are a girl?

Answer 1

A school uniform which would be a grey skirt, white shirt, and a tie (preferably green, red, yellow, or blue) and maybe something around your hair?

You should also try and find a black robe.

Or if you have a full length dress or a maxi you could say you are wearing dress robes. Just make sure you are all done up like you are going to a formal occasion :)

good luck

P.S. MAKE SURE TO HAVE A WAND! A stick would do.