Do you have to go to a school that has a strict uniform policy?
Wear makeup. Don't wear a whole lot of makeup, just wear enough to make your features stand out. Try to go with light colors to make your face look bright and refreshed.
Wear jewelry. Beaded necklaces look good with any color or type of polo, they make the uniform look "classy". Short necklaces go better with polos then longer necklaces. And If you like to wear shorts to school, then ankle bracelets are fun and cute too. Also, lots of bracelets are colorful or you can make it look classy and go with brown or beige bracelets depending on the color of your uniform.
Wear skinny khakis. Walmart sells khakis in the form of skinny jeans, they are really much more cute then regular style baggy khakis or flare out khakis. Plus uggs or any other kind of boot looks good with skinny khakis, assuming you don't have a strict shoe policy.
Never wear the same hair style everyday, it can get boring. Try mixing it up a bit by curling your hair one day and straightening the next, or whatever works best for you.
Remember less makeup is always more
Don't wear house shoes. You don't want to look like you just got out of bed
Make sure to always match up your colors with your uniform. You don't want to look like a clown.