Just get whatever bag you want. I started the year with a backpack I think, but ended the year with one of those thin canvas tote bags. If you have a locker to put your folders/textbooks in, then you don't need anything super sturdy or expensive. Guys at my Sixth Form were rarely seen with bags (most of them didn't even bother with folders/textbooks) but those who did generally went for the Nike backpacks/satchel type bags so you wouldn't look out of place. As long as it can hold your stuff don't worry too much about the design, no-one will really pay that much attention.