Hi - my sixth form is quite strict on the dress code as well. My school don't really mind what you use, it can be any colour, style or pattern, just as long as it's not too insane. It depends how you file your stuff and how much room you want in a bag. You don't have to go to the extremes that some Year 7s do rearding bag size, but having a big bag isn't a bad thing. I would recommend checking out H&M, Topshop, River Island, Vans and Eastpak. There will be a variety of styles sold by each brand and hopefully something which matches your needs and taste.