school library fine, books were taken from the classroom, who pays?

we have a fine for two library books checked out by my 4th grader, I pick him up and check his backpack every day and he never brought the books home

he did check them out but they were in his classroom and then they weren't and the library never got them back

I would just pay it but I'm very low income and over 80% of that income goes to rent

I can make payments and eventually pay it off but is that our responsibility if the books are taken from the classroom? thanks