Um... if it's private, don't put it in a publicly-viewable table, even
if it is encrypted....

Have each student create a public/private key set with gpg.  Then post the
keys on a key server.  You publish the grades by sending them a message
encrypted with the public key.  The student decrypts it with the private
key.

That would, however, be a simply MAHVELOUS class to implement in Ruby.

Ted, long-time member of the American Cryptogram Association.