You can view your Student Codes on the 'Student Codes' tab of your Dashboard.  Here you can see how many usages you have left.  On this tab you can also select a student code to see more information such as: who has redeemed the code, when the code was redeemed and you are able to revoke extra or unauthorized redemptions of your code.  

Check out this article for information on how to revoke a redemption of a student code.