Academic Misconduct

I am an instructor in a Community College and I'm looking for a macro that we can imbed into Microsoft files that will produce a 7 digit code number. This code should be produced with the first save, and then be protected from change or modification by the user, or even done seemlessly so that that student doesn't know it is being done. We will use this information to ensure that there is no passsing of files for assignments etc. Any suggestions would be a great help. Thanks