Depends what rules your sixth form has really
At mine we can only wear black, white or grey and no patterns (which is super shitty) so I normally stick with comfy black skinny jeans and a plain grey jumper/sweater or a plain shirt and hoodie. At the start of the year I wore cute dresses and proper white shirts but its not worth it, might as well wear something comfy! Buy whatever you want and whatever's comfortable for you, everyones gonna wear their own thing so theres little chance you'll look out of place