Per Microsoft's own website the second shot vouchers work with academic exams as well. Sounds like maybe the second shot voucher may not be valid at all? Call Microsoft support or try another vouch number (they are free, just sign up
with your email and try another code).
http://www.microsoft.com/learning/en/us/second-shot-faq.aspx
Q.Which
exams qualify?A.
The Second Shot offer applies to all IT professional and developer certification exams (all exams with a 070 prefix) and Microsoft Dynamics exams. The offer also includes all academic exams (exams with a 072 prefix).