Private Sub BtnGrade_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles BtnGrade.Click
        Dim s1, s2, s3, s4, s5 As Integer
        Dim grade As String
        Dim average As Double
        Dim isconverted As Boolean
        isconverted = Integer.TryParse(TxtS1.Text, s1)
        isconverted = Integer.TryParse(TxtS2.Text, s2)
        isconverted = Integer.TryParse(TxtS3.Text, s3)
        isconverted = Integer.TryParse(TxtS4.Text, s4)
        isconverted = Integer.TryParse(TxtS5.Text, s5)
        If isconverted = True Then
            average = (s1 + s2 + s3 + s4 + s5) / 5
            grade = LblGrade.Text

            Select Case average
                Case Is <= 70
                    grade = "Tiger"
                Case 70 To 74
                    grade = "Pro"
                Case 75 To 90
                    grade = "Want to Be"
                Case Is >= 90
                    grade = "Duffer"
            End Select

            LblGrade.Text = grade.ToString()
        Else
            MessageBox.Show("Wrong Information entered", "Quizz4", MessageBoxButtons.OK, MessageBoxIcon.Error, MessageBoxDefaultButton.Button1, MessageBoxOptions.DefaultDesktopOnly)
        End If
    End Sub


    Private Sub BtnClear_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles BtnClear.Click
        TxtS1.Text = ""
        TxtS2.Text = ""
        TxtS3.Text = ""
        TxtS4.Text = ""
        TxtS5.Text = ""
        LblGrade.Text = ""
    End Sub
End Class

...
Dim sum As byte=0
if val(TxtS1.Text)<>0 sum +=1
if val(TxtS2.Text)<>0 sum +=1
if val(TxtS3.Text)<>0 sum +=1
if val(TxtS4.Text)<>0 sum +=1
if val(TxtS5.Text)<>0 sum +=1
If sum > 1 Then
...

Try
     'your code here to parse values
Catch ex As NumberFormatException
     Console.WriteLine("your message to user")
End Try