Schools enrolled in Apple's Volume Purchase Program for Education can get a 50% discount when buying at least 20 copies of Notability. You can find Notability on the VPP store here:
You can also find a guide to distributing VPP licenses here.
If you'd like to buy Notability for a large district and have questions, we'd love to hear from you! Please feel free to reach out through sales@gingerlabs.com.
At the moment, the App Store doesn't let us offer discounts for individual students and teachers.