Okay I'm gonna tackle these one at a time:
Hair- Most people in my private school have their hair braided or up in an elegant bun.
Clothes- Have an undershirt and unbutton the first few so you don't look like a wrapped up turkey. You can wear decorative, solid colored socks. As for the bottoms I don't know what you have. If it's a skirt great, but if it's pants that's fine too.
Shoes- Cute little flats are always nice. In the case of sneakers the smaller your foot looks the better' so you make sure if you wear a sneaker its slimming.
Make-Up- Now that is a bit harder, but that doesn't mean you need more. People (mainly boys) like it when girls wear less make-up. As for what to put on I suggest eye-shadow, blush, and for lips I suggest lip gloss(littlest of this above all). Only lightly should you apply any make-up and that is VERY important!
Hope I could be of help