how can one register the purchase of a textbook by a school in its journal
You would record by either a Purchase Invoice if payment is being made later or a Spend Money if payment was made at the same time.
Transactions involving the movement of money into or out of the business cannot be entered via journal entries. They can only be entered as bank or cash transactions. See these Guides:
https://www.manager.io/guides/7821
https://www.manager.io/guides/7820