Well it all depends on what your school will allow. Do you have to wear a uniform? Does your outfit have to be a particular colour? Can you wear a skirt?
If you have a uniform it will probably consist of a polo top. If you dont, you can always buy one fairly cheaply and in heaps of different colours. Polo tops are great for sport becasue they are so comfortable. Blair wears hers buttoned up. link
Blair tucks her polo top into a sports skirt, which you can also get in a range of clours and styles from sports shops. Blairs skirt is very short, so you would have to also get maybe short bike shorts to go underneath. link
Blair teams all this with High socks: link
a wide belt: link
And a neat, pulled back hairstyle:link
I dont think you see her shoes, but I think this is the one time when she does not wear heels :]