This store links to your App when you register it. Your app links back to this store when you create a BUY button with the URL below. Copy the URL, then create a hyperlinked buy button on your site. Change the items in red to match your app.
https://docs.google.com/forms/d/1AS9zN5wL7WXOPbN5jaHfhwlY2j5R1Ph0fpX1P-pPhoI/viewform?entry.1286223400=test&entry.141998720=http://test.com&entry.513873670=test@test.com
While editing your form, choose Responses > Get Pre-filled URL
Then fill in the URL, hit submit, and paste the result above for your students--replacing what is here. ALL ITEMS IN RED ARE UNIQUE FOR YOUR CLASS--SO REPLACE WHAT IS HERE.