If a student was accidentally deleted from a teacher's gradebook or the student got deleted because they left and then came back can their scores be recovered?

If a teacher deleted a student from their gradebook when the student has been deleted, a report will become available under the Report History tab on the View All Reports page. This report will print the scores of the deleted student in case they were deleted in error. The scores would need to be manually added back in for the student.

Additionally if a teacher made a backup of their gradebook before the student was deleted they can restore a previous gradebook to get the student's scores. 

Link to documentation on how to restore a gradebook: How to Restore a Deleted Gradebook