What should i wear?

Dark colored skinny jeans look more formal than light colored skinny jeans. How about one of those baby doll tops that are longer - like hits you mid-thigh. It almost looks like a dress. Then wear that with the dark skinny jeans and the flats that you already have. If you don't already have a baby doll top, you can go to Forever 21 and get one pretty cheap. Also, accessorize to finish the look so it's more dressy. Nothing big and gaudy or course but something like a long necklace - the kind with a pendant that dangles around your bellyon. Matching earrings will finish the look. This way, you will get to wear what you like and keep your own style while being dressy enough for such an event.