Each of your students' subscriptions will expire one year from the date that they redeem your student code (unless you have a custom student code that grants subscriptions that last less than a year, i.e. one semester or quarter, etc.).  Your students' subscription expiration date is not necessarily the same as the date that your subscription expires (unless they all redeemed your student code on the same day that your account was activated).  You can view the date that each of your students redeemed your student code by going to the 'Student Codes' tab on your Dashboard and clicking 'View' by the Student Code in question.  You can view each of your students' subscription expiration date by going to the 'Students' tab on your dashboard and looking at the date in the 'Subscription Ends' column.

Check out this article which gives information on how students can renew their subscription.