Hi! Common reasons for this problem include spam servers at schools/email clients and mistakes in the email address used. You should check to make sure those are not issues, and then you can go here to have it resent: http://students.autodesk.com/?nd=register_resend
If you are still having a problem, you may send an email to studentcommunity@autodesk.com - provide your email address, your name, and your school.