Anyway, in my efforts to be a responsible parent, I decided to go with a water-proof, or at least rain proof-ish library bag. Sure, it may not hold up in a torrential downpour but should do a good enough job to keep things dry during the classroom-to-car dash.
The white fabric with red dots is coated in plastic. It is quite flimsy but choices were limited. I fully lined the bag with the red fabric with the white dots, so it is just as pretty on the inside as on the outside. Apart from the flimsiness, it turned out well - except for the strap that is way too long. But if it were any shorter, I wouldn't be able to borrow it!