If you have donated the supplies to the school, (I'm assuming it is a public school) you can enter the Fair Market Value (FMV) of the supplies as your charitable donation. The FMV is a price that you would have received if you had sold the supplies to a willing buyer.
The charitable donations are part of your itemized deductions in the Deductions & Credits section:
- Federal Taxes
- Deductions & Credits
- Scroll down to Charitable Donations
- Select Donations to Charity in 2016 - select Items