Try adding the following macro to your mailmerge main document; when you click on Finish & Merge>Edit Individual Documents, the macro will automatically delete all table rows where the Grade is empty.
Code:
Sub MailMergeToDoc()
Application.ScreenUpdating = False
Dim Tbl As Table, r As Long
ActiveDocument.MailMerge.Execute
With ActiveDocument
For Each Tbl In .Tables
With Tbl
For r = .Rows.Count To 3 Step -1
With .Rows(r)
If Len(.Cells(2).Range.Text) = 2 Then .Delete
End With
Next
End With
Next
End With
Application.ScreenUpdating = False
End Sub