(Disclosure: I work at GitHub)

So long as you’re still a student, you’re eligible for the academic discount. Once your coupon expires, you can re-apply for the student discount.

When you re-apply, be sure you’ve added your school-issued email address to your account, that will expedite the approval process.

You will receive a notification email before your discount expires. When the discount does expire, your account will be locked which means that you won’t be able to access your private repositories. Your private repositories will not be deleted and they will not be made public.

Of course, if you ever have any troubles, you can always email support@github.com.