Is it permissible for a female teacher to receive a percentage from the discount on school books that she arranges to purchase for her students from a specific bookstore, and does the permissibility of that differ between her paying for the books first and then selling them to the students, or taking money from them and then purchasing the books?

If you are an agent for people in purchasing books, it is not permissible for you to take an amount exceeding the price you paid to the bookstore, unless you have agreed with them to take a known amount as an agency fee. It is also permissible for you to buy the books from the bookstore and then sell them to them at an increased price after you have taken possession of the books. There is no objection to borrowing money for the purchase or buying on credit.