Tax-exempt status on app puchases
It's our understanding that iTunes adds tax as required based on your location:
"iTunes 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 iTunes Store."
If you need to confirm this for your location, you can contact iTunes 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's Volume Purchase Program for Education and purchase the license through that program.