The recommendations on the Ortliebs are spot-on. For what you're packing around for school or office work, I've had really good results with the Ortlieb "Downtowner", which is pretty much a waterproof/mostly dustproof briefcase with the Ortlieb QL2 attachment system on the back. There's room for a netbook or full sized laptop in there, as couple books, notebooks, and writing accessories. For commuting, I'll stick my laptop and notebook in there, and then put one of the Rollers or a BikePacker on the other side to hold a change of work clothes.