Use your imagination! A friend of mine did a human Twilight Sparkle illustration- and she was wearing a sundress, as compared to her usual school uniform gear.
I did have a school uniform like cosplay. I put the cutie mark on the front of the knit vest. All 3 pieces- blouse, vest, and skirt- were varying shades of purple and lilac/lavender. I had a little tie in magenta as well, plus argyle socks with her colors. Since I am allergic to wigs and hair color, I simply put ribbons on the headband to reflect her hair colors. I also had 3 ribbons to represent her tail- which is my favorite aspect, as well as the easiest one to make. And I made the horn and ears all by myself.
Pic is in my gallery under My Little Pony cosplay.
I think, therefore I am.