Do you have stamps that look like bags? Or even a present stamp that you can ink up, missing the bow? Then draw a handle on it for a bag?? (I have one in Mini Mates that would be perfect. It looks just like a bag, if you skipped the bow). Then you could draw (practice first!) the spine of a book poking out from the bag?
I guess you don't have to relate it to Book Bags. You could simply do something fun or elegant.....nothing related to books.
Oh, just a thought....what about stamping the bookmark with FrenchScript BG stamp which looks like a page from a book, then bend the top of the cardstock so it looks like a dogeared page....also have the words upside down on the dog ear part.
If you don't have an appropiate stamp, you could always rip out pages from books,then decoupage them onto the bookmark. No stamping involved in that. You could highlight certain words with your markers for fun before you apply the Mod Podge.