You may face an issue where teachers are finding students from prior years are still in their gradebooks and their grades are exporting with this year's records.
This issue occurs when you haven't chosen unique codes for each marking period so the school's marking periods were configured to use the same code from last year.
Configuring Marking Periods
If grades are not submitted yet, you can still change the unique code of the marking period by following the steps in the article Configuring Marking Periods. A school admin can also perform the change by navigating to Grade Export System from the Toolbox in their portal.
Reload From Source
If the grades are already submitted and the term is still not finished, a workaround can be suggested to the teachers to correct the grade export data for this term. Follow the steps in the article Reloading Grades from Source
Changing Unique Codes for Old Marking Periods
If the term is done and you're not capable of performing a reload from the source, you may want to change the codes for the marking periods that have already closed, however, this is not recommended as it could lead to data sync issues. More details can be found in the article Changing Codes for Old Marking Periods