Question: If a student has attendance marks after his leaving date, do I need to remove these before I off roll them?
Answer: Yes, you will need to remove these before you off roll them to prevent any future issues.
To do this go to Register>Manage Register and from the "single student" dropdown, find the student and put in the dates required and click View Register. Now just use the "remove codes" button at the bottom of the table to remove the attendance code.
This student can now be off rolled