I think that a full version will open a Student file but it will switch AC to the Student version and any file opened or created and saved without quitting first will become a student file which is irreversible. You cannot convert a Student File into a Regular File, nor copy-paste information between them.
I don't see any reason to reopen a AC12 student file in a previous version of AC. If you are a student then you can download the AC12 student version also and work on your school project there. If you want to use the student version in an Office Environment then you are violating your license.
What is the reason for opening the AC12 student file in AC8.1?
Eduardo Rolón AIA NCARB AC26 US/INT -> AC08
Macbook Pro M1 Max 64GB ram, OS X 10.XX latest another Moderator