I have values in one of my W2 forms in lines 18 and 19 and box 20 is filled in for Cincinnati. Is this for the school district taxes? If so, where do I input this info?

You must have worked in the city of Cincinnati and your employer withheld tax for the city.  That is not a school district tax.   If you live in OH, there are a lot of towns and cities with their own local taxes so you need to know if you live (or work) in one that has a tax.  

In many instances, you will need to file a local tax return for the area you live in, but show the tax you already paid to the city you worked in.   

TurboTax only prepares a few city or local tax returns as an extra step in your state return.  For most local tax returns you are on your own.  You may have to ask around amongst your neighbors, etc. to find out if you have a local tax and how to pay it. “Google “ local tax for your area.  Call your city government, etc.  Your only record of paying your local tax will be your own bank/credit card records

https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/city-tax-returns-turbotax-state-program/L0fev7XS3_US_en_US?uid=m69jiu1e

These may be cities and towns in Ohio that will require you to file local income tax returns.  TurboTax supports all CCA and RITA returns.

The municipal income tax applies to Individuals who:

• Earn taxable income in one or more Ohio municipalities, and 
• Live in a Ohio municipality.

Individuals owe municipal income tax to the municipality(ies) in which they earn their income, AND may also owe municipal income tax to the municipality where they live.

Individuals may be subject to two municipal taxes:

• Workplace – tax where earned by taxpayer, and
• Residence – tax where taxpayer resides

See here and here.