I think you are wanting to start a new Grade sheet for each class.
Rename or delete Grade sheet. Rename if you want to keep the information, otherwise delete.
Duplicate the Student Response sheet if you want to maintain the history.
If you duplicate, consider renaming.
Delete all of the rows in the Student Submissions except rows that contain the heading, Hints (if used) and the answer key.
At this point, you can setup autograde if you would like.
Regrade the Student Submissions as desired. Flubaroo will create a new Grade sheet.
Repeat the above steps for each new class.