In a recent episode, Alastair mentioned that he thought we never get any indication that Hogwarts students carry around bags of, e.g., books. However, I think we do get multiple indications of such.
In Chamber of Secrets, I believe Harry's bag rips, causing the contents, including Tom Riddle's diary, to spill out in the hallway. At that moment, Harry discovers that the diary appears impervious to all the spilled ink in his bag, which gives him an idea about how to engage Tom Riddle. I believe that's also the moment at which Ginny discovers that Harry has the diary.
In Prisoner of Azkaban, I believe there are multiple references to Hermione's bag nearly bursting at the seams due to being stuffed with so many books.
It's possible I'm misremembering - what the rest of you think?