The school makes a purchase of new books from a publisher. They select the "modern teen reading" package, and pay a lump sum for 100 books across nine categories. Some of the books under investigation are in the received package. What was the cost of the individual books under investigation?
Ooh, I know this one - if acquired as a non-itemised purchase, the valuation should be the number of books we care about, divided by the total number of books (100 in this case), multiplied by the total lump sum paid.
However, you acquired three identical items. The lump sum changes each time. You are not sure which of five different purchases these items belong to. How do you determine how much was paid?
I agree that there are some complications, but it is a solved problem, with a myriad of off the shelf asset tracking software solutions. It does add a bit of effort to the unpacking of a shipment, but not beyond what's expected of any retailer.
A school, however, is not a retailer. They likely have not done those things in the past, and so the simplest solution for them is to not do this extra work, and remove the offending items from their shelves.