You should have an account with Adobe and in that account there should be a record of your programs and the serial numbers used to activate them.
The student edition should not be any different from any other, so you should just need to download the trial version of the software and then activate it using your serial number.
You can download the trial version of the software using the link below and then use your current serial number to activate it.
Be sure to follow the steps outlined in the Note: Very Important Instructions section on the download pages at this site or else the download will not work properly.
CS5: http://prodesigntools.com/all-adobe-cs5-direct-download-links.html
If you should have any problems with the serial number or activation, you need to contact Adobe Support either by chat or via phone.
Here are some links to help make contact:
http://www.adobe.com/support/chat/ivrchat.html