You will need to mark pupils who leave as out of use in Tali before importing new and updated pupil details.

Select Administration > Manage Names.
Use the Filter field to specify the year which identifies your leavers.
Select ALL the names in the year:
Click the box next to the column header Name to select all items on a page.
Do this for each page of names.
In the Batch Actions section:
Click Out of Use.
Click Update Names.
The Out of Use names appear in grey italics.

Using MISapp

For any users of MISapp you can easily identify and mark out of use any School Leavers. To do this:

Select Administration > Manage Names.
Click the Updated Names tab.
Enter the Date from (leave the Date to which defaults to tomorrow).
Click Search.
Click Deleted.
Check all the names on all the pages to confirm they have left and then click the box against their name to select them.
Click Mark name(s) as out of use.