When a pupil leaves the school you can archive them in Insight. Once archived, a pupil won't be included in cohort reports, but you'll still be able to view their individual report page (just search for them by name in the search bar).
How to Archive Pupils who've Left the School
You can run the Update Pupils process at any time to sync your pupil roll from your MIS. This will automatically archive any children who are no longer in the school.
You can also manually archive a pupil. Go to Admin then Manage Pupils:
Search for a pupil by name and press Search:
Click Edit next to the pupil you want to archive:
Scroll down and add the date the pupil left the school into the End date field, then press Save Changes: