If you log in to your SAFE Portal, and go to Orders and Payments, is there a button at the bottom to 'Enter a code'? If so, you may be able to use that with your student subscription key.
Only 'buy more' and 'change payment method' options on that page.
We've had this many times before, and I still don't know what the solution is, as no one ever posts here once the issue is sorted. All I can suggest is that you raise a Support Ticket.