You only input *ONE* grade, before the loop.
You need to get one grade EACH TIME through the loop.
And though your IF tests work, you can make it simpler if you use ELSE, as well.
Write “Input one grade"
If InputScore < 0 Then breakOutOfLoop /* how you break out depends on language*/
If (InputScore >= 90) Then
Else If InputScore >=80) Then
Else If InputScore>=70 Then
Else If InputScore>=60 Then