aeropostale camis in various colors are awesome. they can go under anything, or just layer a few for warmer days. id say get about 5 pairs of basic, boot cut or straight leg jeans. they really come in handy. then you can buy a couple pairs with extra designs for the occasional dress up.
short sleeve shirts should dominate your back to school wardrobe. in the winter, you can wear them (or just camis) under hoodies, sweaters, cardigans, or jackets. long sleeve ones are less versatile, but get a few fitted ls tees, which are also easy to layer.
you might want to invest in some collared shirts as well. they come in handy when you least expect it. schooldays, concerts, dinners, whatever. you can easily dress them up or down for any occasion.
as for shoes, one pair of tennis shoes is enough. but get a couple pairs of flats in easy-to-match colors, like silver & brown. be sure you can easily walk up 3 flights of stairs in them. some cute, casual boots for the winter are good if you can afford them. and of course flip flops for summer or dress-down days. i recommend the fabric-strap ones versus rubber or plastic. they can be worn longer and are wayy more classy.
once youve got the basics, you can spend any extra money on accessories that you'll use often, like necklaces of various lengths & colors, a couple totes (avoid white, it gets dirty easily), and maybe some hair things too. good luck, & have fun!!