If the performance category was not deleted in Registrar Setup, please refer to  No students receive performance categories when running a Performance Category calculation and/or Student incorrectly receives a performance category level .

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Deleting a Performance Category in Registrar Setup, will bring up the message" Deleting this performance category will delete all performance levels awarded to students from this category. Are you sure you want to delete this performance category?"  By clicking Yes on the message above, the category would be removed from all students' records.