Tax-exempt status on app puchases
From the App Store Terms:
"App Store purchases will include sales tax based on the bill-to address and the sales tax rate in effect at the time your purchase is completed. If the sales tax rate for the billing address changes before the purchased digital content is downloaded, the new tax rate in effect at the time of download will apply. We will only charge tax in states where digital goods are taxable. No customers are eligible for tax exemptions for purchases made on the App Store."
If you need to confirm this for your location, you can contact Apple Support.
If you are a tax-exempt educational institution and would like this status to be applied to App Store purchases, you'd need to sign up with Apple School Manager and purchase licenses through that program.