The thought processes of a child first being able to dress herself are complex and really quite sensible. Oddly enough, most of these premises continue to effect the way I dress today...

How to be taken seriously:
-Wear shoes that look good but aren't too new. A little dirt gives you street cred.
-Wear clothes that allow you to keep up with the boys. Never a skirt without shorts! Those boys cannot have the monkey bars all to themselves!!
-Make sure your backpack is appropriately cool. I was dissuaded from character branding whenever possible. My only "theme" backpack was Beauty & the Beast in first grade. Solid colors were definitely ingrained.
-Play hard in your clothes, but don't look like a ragamuffin. Parents don't let strange mud-covered children stay for dinner.

How NOT to be taken seriously:
frivolous patterns, ill-fitting or worn-out clothing, poor hygiene...
and the #1: pigtails.
You simply cannot be taken seriously in any context if your hair is in pigtails. Braided, curled, straight, flippy, low, high, short, spiky, flowy, poofy... pigtails are neither intimidating nor impressive. If drama arises on the playground, you will immediately regret wearing pigtails. They are a tool for your opponent to drag you around by, and they just aren't tough. A braid is a good long-haired alternative to pigtails or a ponytail as it looks well-taken-care-of but also keeps your hair out of your face. Also, a good weapon for hitting people with.