I think this is the answer for which you're looking:

Any contribution involving the payment of money to or on behalf of a charity is considered a cash contribution, whether it is in the form of cash, check, credit card, payroll deduction, out-of-pocket expense, student living expense, or foster parent  expense. For example, if the taxpayer purchases food or new clothing using cash, check, or a charge card and donates these items to the charity, this donation is considered a cash donation. A noncash contribution is a donation of property owned by the taxpayer. [Source: The TaxBook, pg 4-16]

If the clients purchased the school supplies specifically for the school and donated them to the school, it's the equivalent of a cash donation. If the clients already owned school supplies which they contributed to the school, I would use the generic (a)  Household items & clothing.