My employer has been taking out the wrong school district taxes since 2017. How can i get a refund for the school district i have not been living in?

What state?

In Ohio, you simply file a form SD100, for each year,  to the Ohio Dept of Taxation, indicating that you were not a resident of that district.  Attach a copy of the W-2 showing the withholding.

The TurboTax Ohio software can do form SD100.  

It may be too late to get a refund for 2017, but I would try filing one anyway. 

If you live within a different school district (SD), with an income tax, you'll have to file a separate SD100 to pay tax to that district.  Ohio SDs do not credit for tax withheld by the wrong district.  

To verify or find out if you live in a school district or city with an income tax enter your address at:

http://www.tax.ohio.gov/Individual/LocalTaxInformation.aspx

To get Ohio paper forms, see https://tax.ohio.gov/wps/portal/gov/tax/individual/get-a-form