Box 19 of the W-2. Box 20 should identify the school district (SD) that the $ was withheld for by name or number. When you enter it in TurboTax, type 1404 in box 20.
Some employers mistakenly put SD withholding in box 14; so look there if it's not in 19.
SD withholding is not mandatory; so it's possible there was none