During student information system syncing, the default behavior is to sync students with imports. This means that students who are not part of a new import will automatically be deleted from the system to maintain parody with your SIS. This, however, is not always ideal and can be overridden on a per-student basis from the administration portal.
Excluding a Student from Syncing
When adding or editing a student from the administration portal, just check the box for "Exclude student from SIS sync". That's it! Now as data is synced from your SIS, any student with this checked will be retained.